NAME
Data::HashMap::Shared - Multiprocess shared-memory hash maps with LRU eviction and per-key TTL
SYNOPSIS
use Data::HashMap::Shared::II;
# Create or open a shared map (file-backed mmap)
my $map = Data::HashMap::Shared::II->new('/tmp/mymap.shm', 100000);
# Keyword API (fastest)
shm_ii_put $map, 42, 100;
my $val = shm_ii_get $map, 42;
# Method API
$map->put(42, 100);
my $v = $map->get(42);
# Atomic counters (under the read lock, without LRU or TTL)
shm_ii_incr $map, 1; # 1
shm_ii_incr_by $map, 1, 10; # 11
shm_ii_max $map, 1, 50; # monotonic: store max(current, 50) -> 50
# Compare-and-swap (all variants; byte-compare for string values)
shm_ii_cas $map, 1, 50, 42; # swap to 42 only if current == 50
# LRU cache (evicts least-recently-used when full)
my $cache = Data::HashMap::Shared::II->new('/tmp/cache.shm', 100000, 1000);
shm_ii_put $cache, 42, 100; # auto-evicts LRU entry if size > 1000
# TTL (entries expire after N seconds)
my $ttl_map = Data::HashMap::Shared::II->new('/tmp/ttl.shm', 100000, 0, 60);
shm_ii_put $ttl_map, 1, 10; # expires in 60s
shm_ii_put_ttl $ttl_map, 2, 20, 5; # per-key: expires in 5s
# Multiprocess
if (fork() == 0) {
my $child = Data::HashMap::Shared::II->new('/tmp/mymap.shm', 100000);
shm_ii_incr $child, 1; # atomic increment visible to parent
exit;
}
wait;DESCRIPTION
Data::HashMap::Shared provides type-specialized hash maps stored in file-backed shared memory (mmap(MAP_SHARED)), enabling efficient multiprocess data sharing on Linux. With opt-in LRU eviction and per-key TTL it doubles as a fast cross-process cache; lookups take a lock-free seqlock fast path.
Linux-only. Requires 64-bit Perl.
Features
File-backed mmap for cross-process sharing
Futex-based read-write lock (fast userspace path)
Atomic counters (incr/decr under the read lock on maps without LRU or TTL)
Elastic capacity (starts small, grows/shrinks automatically)
Arena allocator for string storage in shared memory
Keyword API via XS::Parse::Keyword for maximum speed
Opt-in LRU eviction -- clock/second-chance algorithm; reads stay lock-free
Opt-in per-key TTL expiry -- lazy removal on access; monotonic clock
Stale lock recovery for both writers and readers (dead PIDs detected and drained automatically)
Variants
Integer Range and Wrapping
Integer keys and values are stored as fixed-width two's-complement integers: I16/SI16/I16S use a signed 16-bit range (-32768 .. 32767), I32/SI32/I32S a signed 32-bit range, and II/IS/SI a signed 64-bit range. A key or value outside the variant's range is silently truncated to the low bits (two's complement), with no warning: on an I16 map, $map->put(70000, ...) stores under key 4464 (70000 & 0xFFFF), so get(70000) and get(4464) address the same entry. incr/decr wrap the same way (32767 + 1 becomes -32768). Pick a variant wide enough for your data.
Constructor
my $map = Data::HashMap::Shared::II->new($path, $max_entries);
my $map = Data::HashMap::Shared::II->new(undef, $max_entries); # anonymous
my $map = Data::HashMap::Shared::II->new($path, $max_entries, $max_size);
my $map = Data::HashMap::Shared::II->new($path, $max_entries, $max_size, $ttl);
my $map = Data::HashMap::Shared::II->new($path, $max_entries, $max_size, $ttl, $lru_skip);
my $map = Data::HashMap::Shared::SS->new($path, $max_entries, 0, 0, 0, $arena_cap); # explicit arena bytes
my $map = Data::HashMap::Shared::II->new($path, $max_entries, $max_size, $ttl, $lru_skip, $arena_cap, $file_mode);
my $map = Data::HashMap::Shared::II->new_sharded($prefix, $shards, $max_entries, $max_size, $ttl, $lru_skip, $arena_cap, $file_mode);
my $map = Data::HashMap::Shared::II->new_memfd($name, $max_entries, ...); # memfd-backed
my $map = Data::HashMap::Shared::II->new_from_fd($fd); # reopen memfd
my $fd = $map->memfd; # -1 if not memfdCreates or opens a shared hash map backed by file $path. Passing undef as the path creates an anonymous MAP_SHARED|MAP_ANONYMOUS mapping that is inherited across fork but has no filesystem presence.
new_memfd creates an unlinked memfd-backed map whose file descriptor can be passed to another process (via SCM_RIGHTS, fork+exec, or duped+open). new_from_fd reopens such a descriptor. The descriptor you pass is duplicated (F_DUPFD_CLOEXEC), so it stays yours to close and closing it does not disturb the handle. Both require a 64-bit Perl on Linux (memfd_create(2)).
$map->memfd goes the other way: it returns the handle's own descriptor, not a copy. Pass it, but do not close it: the handle closes it when the map goes away, and in between the number is reissued to the next file the process opens, so an early close makes the map's own close hit an unrelated file. Dup it first if it has to outlive the map.
$max_entries, $max_size, $ttl, $lru_skip, $arena_cap, and $file_mode are used only when creating a new file; when opening an existing one, all parameters are read from the stored header and the constructor arguments are ignored -- but still range-checked, so a value the module would never accept at creation is rejected either way.
$shards is the exception: a set created by 0.20 or later records its count in every shard, so opening it with a different one croaks, and an older set records no count at all. Nothing is ever unlinked, so a refused open leaves behind the shard files it had to create to notice: treat the count as part of the path and the files as one unit, and see "Sharding". Multiple processes can open the same file simultaneously. Dies if the file exists but was created by a different variant or is corrupt.
Optional $max_size enables LRU eviction: when the map reaches $max_size entries, the least-recently-used entry is evicted on insert. Set to 0 (default) to disable. LRU uses a clock/second-chance algorithm.
A $max_size at or above the slot count -- 2048 for a map created with 1000, not the 1536 max_entries reports -- can never drive eviction. The map then fills up and refuses further inserts, keeping its oldest keys -- or, with a $ttl, reclaims expired slots for them instead and so keeps its newest. Every constructor that opens a map for writing warns about this (category misc), reading the bound off the map rather than its arguments, so attaching to a sound file never warns however its arguments are written; silence it with no warnings 'misc'.
Eviction is driven by the entry count, but an exhausted arena evicts too: on a string variant the arena can run out while the count is still below $max_size, and rather than refuse the insert the map evicts one entry and retries the store. Blocks are power-of-two size classes that are never split or coalesced, so the retry still fails when none of the oldest entries holds a block of the class the request needs. A request larger than the whole arena is refused without evicting; one merely larger than every class present is not, and an insert storing both a string key and a string value can evict for each. Size $arena_cap for what you actually store, keep the sizes within a few classes, and check what the insert returned. A map without $max_size has nothing to evict and still fails the insert. Overwriting a key that is already there -- put on a hit, update, swap, cas -- stores the new value before releasing the old one, so on a full arena it evicts the same way, never the entry it is replacing; without $max_size it fails and leaves the entry as it was, even when the replacement is the same size as the value it replaces.
Keys and values of 7 bytes or fewer are stored inline and need no arena at all. A single key or value may be just under 1 GB; longer ones croak.
Optional $ttl sets a default time-to-live in seconds for all entries. Expired entries are reclaimed lazily, by the next mutating access to that key. remove, update, take, cas, touch, persist and set_ttl free the slot; incr, add and get_or_set free it and insert afresh, so the key is live again when they return; put overwrites it in place. A read -- get, exists, get_with_ttl, get_multi, ttl_remaining -- reports it absent but leaves it there, so size still counts it. Set to 0 (default) to disable.
TTLs have whole-second granularity and the deadline is truncated, so an entry given $n seconds expires somewhere between $n-1 and $n seconds later: a TTL of 1 can expire almost immediately. A refresh that has to beat the TTL needs more than a second of margin: a heartbeat refreshed every second wants a TTL of 3, not 2.
Any operation that stores a value resets the entry's TTL to the map default: put, update, cas, incr/decr/incr_by, max/min, get_or_set on a hit, and set_multi, as well as the documented touch and swap. A permanent entry (TTL 0) stays permanent. To carry a per-key TTL across a value change, write it with put_ttl/update_ttl or restore it afterwards with set_ttl.
An expired entry keeps its slot until something reclaims it, so size counts entries that every read reports as absent and that keys and the iterators skip. An insert that finds no free slot flushes every expired entry at once rather than failing, so a map whose keys never repeat -- a rate limiter, a dedup guard -- does not wedge once the table fills. It still carries the dead weight until then: flush_expired (or flush_expired_partial on a timer) keeps size honest and the probes short, and $max_size reclaims slots by eviction as well. Expiry is measured against a monotonic clock (CLOCK_MONOTONIC_COARSE): TTLs track elapsed running time and do not advance while the system is suspended or hibernating.
That clock is local to the current boot: it restarts at zero on reboot and is unrelated between machines, while the expiry timestamps live in the file. A map that outlives the boot which wrote it -- or is copied to another host, a frozen map shipped elsewhere included -- therefore carries deadlines on a timeline that no longer exists: entries live longer than their TTL where the destination's uptime has not yet reached the stored values, and arrive already expired where it has, while size still counts them. Nothing crashes, but do not rely on TTL across a reboot or a host move: flush_expired, persist what you ship, or rebuild.
Optional $lru_skip (0-99, default 0; 100 or more disables skipping, a negative value is rejected like any out-of-range size) reduces how often LRU promotion reorders the recency list -- higher values skip more. Promotion runs only where an operation updates an existing entry under the write lock (put/incr/get_or_set on a hit, the update family); get, get_with_ttl and get_multi never promote, they set the lock-free accessed bit that clock eviction consumes; exists sets neither, so a key probed only with exists is evicted as though it had never been read. Skipping cuts write-lock churn on Zipfian workloads where a few hot keys dominate. The eviction victim itself is never skipped, so eviction stays correct at any setting. Set to 0 for strict LRU ordering.
Optional $arena_cap (bytes) sizes the string arena explicitly instead of deriving it from $max_entries. The default is roughly 128 bytes per entry (4096 minimum), which a few large strings can exhaust while the table is nearly empty. Clamped to [4096, 0xFFFFFFFF]; integer-only variants (II/I16/I32) have none and ignore it; for sharded maps it is per shard, like $max_entries.
Size it from the rounded lengths, not the byte totals: blocks are powers of two with a 16-byte minimum, so a 100-byte value takes 128 and a 1100-byte value 2048, and a total can be short by up to half. Blocks are recycled by exact size class, so a freed large one never serves a smaller request -- a workload alternating value sizes needs room for one block of each size it uses, not just the largest.
Optional $file_mode (octal, default 0600) sets the permission bits used when the backing file is created; the exact mode is applied via fchmod, so the process umask does not narrow it. It is ignored when attaching an existing file and for anonymous or memfd-backed maps. Pass a wider mode such as 0666 to opt in to cross-user sharing. Before version 0.14 the default was 0666.
Zero-cost when disabled: with both $max_size=0 and $ttl=0, the fast lock-free read path is used.
String Keys/Values and UTF-8
String-key variants (SS, SI, SI16, SI32) compare keys as raw bytes: two keys are the same entry if and only if they contain the same byte sequence. The SV UTF-8 flag is stored alongside the key so retrieval round-trips it to the returned SV, but it is not part of key identity. Consequences:
ASCII keys with a toggled UTF-8 flag hash and match the same entry (
use utf8,utf8::upgrade, andutf8::downgradeon ASCII are all equivalent from the map's point of view).Non-ASCII keys with different byte encodings are distinct.
"caf\xe9"(latin-1, 4 bytes) and the same character sequence underuse utf8("caf\xc3\xa9", 5 UTF-8 bytes) are two different keys. If your input comes in mixed encodings, normalize withEncode::encode_utf8before use.
Two keys that a Perl hash cannot tell apart cannot both survive to_hash. Perl downgrades a hash key whose characters all fit in a byte, so a stored key of "caf\xc3\xa9" carrying the UTF-8 flag becomes the hash key "caf\xe9" and lands on top of a stored key of those four bytes. The map holds both, and keys, each and cursors return both entries; the hashref has one, and keys %$h is then smaller than size. Normalize the encoding of your keys, or avoid to_hash when they are mixed.
The stored key keeps the flag it had when the entry was first inserted: a later put with the same bytes and the opposite flag replaces the value but not the key, so keys reports the original flag.
String-value variants (SS, IS, I16S, I32S) store the SV UTF-8 flag alongside each value and round-trip it on retrieval. The cas comparison of $expected against the stored value is byte-only -- the UTF-8 flag on $expected is ignored.
Sharding
my $map = Data::HashMap::Shared::II->new_sharded($path_prefix, $shards, $max_entries, ...);Creates $shards independent maps (files $path_prefix.0, $path_prefix.1, ...) behind a single handle, each with up to $max_entries entries (each sized as if it were a map of its own -- see max_entries below for what the total comes to). Per-key operations automatically route to the correct shard via hash dispatch. Writes to different shards proceed in parallel with independent locks. new_sharded requires a filesystem $path_prefix; anonymous (undef-path) sharded maps are not supported.
The batch ops (set_multi, get_multi, remove_multi) dispatch each key to its shard independently, so on a sharded map a batch is not atomic across shards (the "single lock" note in the API below applies to non-sharded maps).
keys, values, items and to_hash go the other way: they hold every shard's read lock for the whole call, so they alone see a consistent snapshot across shards, and a writer to the first shard is blocked until the last shard has been copied. each takes one shard lock at a time and so blocks a writer only briefly, but crosses shards unsynchronised.
All operations work transparently on sharded maps: put, get, remove, exists, add, update, swap, take, incr, max, min, cas, cas_take, get_or_set, put_ttl, add_ttl, update_ttl, touch, persist, set_ttl, keys, values, items, to_hash, set_multi (method only), remove_multi (method only), get_multi (method only), get_with_ttl (method only), each, pop, shift, drain, clear, flush_expired, flush_expired_partial, size, stats (method only), reserve, and all diagnostic keywords.
Diagnostic counters and capacities reported for a sharded handle are aggregate totals across all shards: size, capacity, max_entries, max_size, tombstones, mmap_size, arena_used, arena_cap, and the stats eviction/expiry/recovery counts all sum over the shards. (ttl is the shared per-entry default, so it reports a single shard's value.) reserve $n pre-grows each shard to $n entries (not $n in total).
Every shard file must come from the same configuration. Opening a set whose files disagree -- one left behind by an earlier run with a different $ttl or $max_size -- croaks, naming the file and the field. Shard 0 is the reference; $max_entries, $max_size, $ttl, $lru_skip, $arena_cap and the routing scheme are compared. The shard count is checked separately, against the $shards you passed: a set written by 0.20 or later records the count it was created with, so opening it with a different $shards croaks instead of silently routing to the wrong files. A set written earlier carries no recorded count: opening one with too few shards is undetected and silently hides every key that routes elsewhere, while too many is caught only because the new shards disagree on the routing scheme -- after the first of them has been created.
Because a missing shard is created fresh, it is created from the arguments this call passed rather than the ones the set was made with; if they differ the set is refused, and refused again on every later open with either set of arguments until the odd shard is removed.
A shard file that goes missing is not detected: the shard is recreated empty and adopted, so the set silently loses every key that routed to it and size drops to match, while a process still holding the set open keeps seeing them. Treat the files as one unit -- copy, move and remove them together. (A set from before 0.20 is refused instead, because the recreated shard disagrees about routing.)
Routing takes the high half of the key's 64-bit hash and slot placement the low half, so the two never compete for the same bits. Sets created before 0.20 routed on the low half -- the bits the probe also uses -- which lengthened probe runs as the shard count grew. Which scheme a set uses is recorded in it, so an existing set keeps working unchanged; it does not gain the shorter probes. Recreate a set to pick them up. Writes with many shards get faster; reads on small-table sets with many shards can be a little slower. Use the smallest shard count that relieves your lock contention, not the largest you can afford.
An earlier release does not refuse a sharded set written by 0.20: the routing scheme is recorded in a byte it does not read, so it opens the set, routes on the low half and misses most of the keys -- and if it writes, it stores each of those keys a second time, in the shard its own scheme picks. keys then lists the key twice and size counts both, each release reads back only its own copy, and a remove from one leaves the other behind.
A shard file the earlier release creates is worse: it records the old scheme, so every open by 0.20 refuses the whole set, naming one offending file at a time. Removing the named files does not repair the set, whose keys were stored under both schemes: rebuild it through items, which walks every shard whatever scheme stored the keys, into a set created by one release. Upgrade every process that shares a sharded set together, as the crash-safety notes already require, and create the set with one release before any mixed fleet runs.
max_entries reports the entry count at the table's 75% design load, which is three quarters of the maximum slot count and so is neither the constructor argument nor the slot count -- a map created with 1000 reports 1536, over 2048 slots. It is not a hard ceiling either: the table grows to the next power of two at or above that reported figure, and inserts keep succeeding until every slot is occupied. Probe length grows sharply over the last few percent, though: a miss on a table at 99% costs roughly an order of magnitude more than at 95%, and one on a completely full table has to walk every slot before it can report absence. Treat max_entries as the size to run at, not the size to reach.
The table shrinks back to fit once removals leave it sparse, undoing any reserve, so a map that is drained and refilled in cycles regrows through every doubling on each refill, each one a full rehash. reserve after each drain avoids it.
Cursors chain across shards automatically. cursor_seek routes to the correct shard based on key hash. $shards is rounded up to the next power of 2.
API
Replace xx with variant prefix: i16, i32, ii, i16s, i32s, is, si16, si32, si, ss.
my $ok = shm_xx_put $map, $key, $value; # insert or overwrite
my $ok = shm_xx_add $map, $key, $value; # insert only if key absent
my $ok = shm_xx_update $map, $key, $value; # overwrite only if key exists
my $old = shm_xx_swap $map, $key, $value; # put + return old value (undef if new)
my $ok = shm_xx_cas $map, $key, $expected, $desired; # compare-and-swap
my $v = shm_xx_cas_take $map, $key, $expected; # compare-and-remove; returns value on match, undef otherwise
my $n = $map->set_multi($k, $v, ...); # batch put under single lock, returns count
my $n = $map->remove_multi(@keys); # batch remove under single lock, returns count
my @v = $map->get_multi($k1, $k2, ...); # batch get under single lock with prefetch pipeline
my ($v, $ttl) = $map->get_with_ttl($key); # atomic snapshot; () if missing, $ttl is undef on non-TTL map, 0 = permanent; sets LRU clock bit
my $v = shm_xx_get $map, $key; # returns undef if not found
my $ok = shm_xx_remove $map, $key; # returns false if not found
my $ok = shm_xx_exists $map, $key; # returns boolean
my $s = shm_xx_size $map;
my $m = shm_xx_max_entries $map;
my @k = shm_xx_keys $map;
my @v = shm_xx_values $map;
my @items = shm_xx_items $map; # flat (k, v, k, v, ...)
while (my ($k, $v) = shm_xx_each $map) { ... } # auto-resets at end
shm_xx_iter_reset $map;
shm_xx_clear $map;
my $href = shm_xx_to_hash $map;
my $v = shm_xx_get_or_set $map, $key, $default; # returns valueSeveral calls below fail for want of room. No room means the table is full (every slot occupied -- see capacity) or, on a variant with string keys or values, the arena is.
get_or_set returns the existing value, or stores and returns $default when the key is absent; undef only when the key is absent and there is no room.
cas, available for all variants, returns true when the stored value matched $expected and was atomically replaced with $desired; false if the key is missing or expired, the value did not match, or there is no room. See "String Keys/Values and UTF-8" for the byte-only comparison rule.
swap returns the previous value, or undef when the key did not exist -- and also undef when there is no room, in which case an existing key keeps its old value. It therefore cannot by itself tell a fresh insert from a failure; check exists or size first if that matters. On a TTL map it refreshes an existing entry's TTL to the default and assigns the default on insert, leaving a permanent entry (TTL 0) permanent.
get_multi returns one element per key, in the order asked, with undef where the key is missing or expired: it never compacts, so the result lines up with the key list. Like get it sets the LRU accessed bit on a hit and leaves an expired entry in place for size to count.
Integer-value variants also have:
my $n = shm_xx_incr $map, $key; # returns new value
my $n = shm_xx_decr $map, $key; # returns new value
my $n = shm_xx_incr_by $map, $key, $delta;
my $n = shm_xx_max $map, $key, $desired; # store max(current, desired), return it
my $n = shm_xx_min $map, $key, $desired; # store min(current, desired), return itA missing key is created starting from zero (Redis-style): the first incr returns 1, decr returns -1, and incr_by returns $delta. These die only when the key is new and there is no room for it. The result wraps at the variant's integer width (see "Integer Range and Wrapping").
max/min atomically store max($current, $desired) / min($current, $desired) and return the resulting value; a missing key is inserted as $desired. Against a concurrent incr_by/cas/max/min on the same key the result is monotonic (max never lowers, min never raises) and never clobbers a concurrent increment. On a map with LRU or TTL every call takes the write lock, promotes the entry in the LRU order and refreshes its TTL even when it stores nothing. Like incr_by, they die only when the key is new and there is no room for it, and the result wraps at the variant's integer width.
LRU/TTL operations (put_ttl, add_ttl, and update_ttl require a TTL-enabled map):
my $ok = shm_xx_put_ttl $map, $key, $value, $ttl_sec; # per-key TTL (0 = permanent); requires TTL-enabled map
my $ok = shm_xx_add_ttl $map, $key, $value, $ttl_sec; # insert-if-absent with per-key TTL (0 = permanent)
my $ok = shm_xx_update_ttl $map, $key, $value, $ttl_sec; # overwrite-only with per-key TTL (0 = permanent)
my $ms = shm_xx_max_size $map; # LRU capacity (0 = disabled)
my $t = shm_xx_ttl $map; # default TTL in seconds
my $r = shm_xx_ttl_remaining $map, $key; # whole seconds left, rounded up (0 = permanent, undef if missing/expired/no TTL)
my $ok = shm_xx_touch $map, $key; # refresh TTL to default (permanent entries stay permanent); promotes in LRU; false if no TTL/LRU
my $ok = shm_xx_persist $map, $key; # remove TTL, make key permanent; false on non-TTL maps
my $ok = shm_xx_set_ttl $map, $key, $sec; # change TTL without changing value (0 = permanent); false on non-TTL maps
my $n = shm_xx_flush_expired $map; # proactively expire all stale entries, returns count
my ($n, $done) = shm_xx_flush_expired_partial $map, $limit; # gradual: scan $limit slots, minimum 1 ($limit per shard on sharded maps; $done true once every shard completes a cycle)Atomic remove-and-return:
my $v = shm_xx_take $map, $key; # remove key and return value (undef if missing)
my ($k, $v) = shm_xx_pop $map; # remove+return from LRU tail / scan forward
my ($k, $v) = shm_xx_shift $map; # remove+return from LRU head / scan backward
my @kv = shm_xx_drain $map, $n; # remove+return up to N entries as flat (k,v,...) listpop and shift remove from opposite ends: pop takes the LRU tail (oldest / least recently used) while shift takes the LRU head (newest / most recently used). On a sharded map they walk the shards in turn and take from each shard's own end, so a sequence of pops is not in global recency order. On non-LRU maps, pop sweeps the slots forward and shift backward, each resuming where its last call stopped and wrapping, so successive partial drains thin the whole table rather than always taking the same end of it. drain removes in pop order (tail-first). pop, shift and drain return an empty list on an empty map, so while (my ($k, $v) = shm_xx_pop $map) ends by itself.
Cursors (independent iterators, allow nesting and removal during iteration):
my $cur = shm_xx_cursor $map; # create cursor
while (my ($k, $v) = shm_xx_cursor_next $cur) { ... }
shm_xx_cursor_reset $cur; # restart from beginning
my $ok = shm_xx_cursor_seek $cur, $key; # position at key (best-effort across resize); true if found, false if missing/expired
# cursor auto-destroyed when out of scope
$cur->next; $cur->reset; $cur->seek($key); # method formsshm_xx_each is also safe to use with remove during iteration.
A cursor_seek that returns false leaves the cursor where it was: it neither repositions a sharded pass nor rewinds a cursor that has run out.
Leaving an each loop early -- last, return, an exception -- leaves the built-in iterator open on that handle, and tombstone compaction and shrink stay deferred for as long as it is: a long-lived handle that keeps removing and re-inserting keys then grows its table instead of compacting it, all the way to its maximum slot count. Removals on their own leave tombstones without growing it. Unlike Perl's each, keys does not reset it. Call iter_reset when you abandon a pass, or run it to completion. The deferral is also per handle, not per map, so another process can compact or shrink the table underneath your iteration; that restarts it, and an abandoned pass can then yield keys it has already returned. Tombstone compaction and shrink are deferred until iteration ends. Growth is not -- a load-driven insert still resizes -- and neither is compaction once the table has reached its maximum capacity and its load (live entries plus tombstones) has passed 75% of the slots: an insert during an iteration can restart it there too, and keys already visited are visited again. On a sharded map a cursor restarts only within the shard it has reached, so shards it already passed are not revisited; take a fresh cursor after a clear if you need a complete pass.
Diagnostics:
my $cap = shm_xx_capacity $map; # current table capacity (slots)
my $tb = shm_xx_tombstones $map; # tombstone count
my $au = shm_xx_arena_used $map; # arena high-water mark (0 for int-only)
my $ac = shm_xx_arena_cap $map; # arena total capacity (0 for int-only)
my $sz = shm_xx_mmap_size $map; # backing file size in bytes
my $ok = shm_xx_reserve $map, $n; # pre-grow (false if exceeds max)
my $ev = shm_xx_stat_evictions $map; # cumulative LRU eviction count
my $ex = shm_xx_stat_expired $map; # cumulative TTL expiration count
my $rc = shm_xx_stat_recoveries $map; # cumulative stale lock recovery count
my $p = $map->path; # backing file path (method only)
my $s = $map->stats; # hashref with all diagnostics in one call (not an atomic snapshot)
# stats keys: size, capacity, max_entries, tombstones, mmap_size,
# arena_used, arena_cap, evictions, expired, recoveries, max_size, ttl,
# frozen, readonlyAn eviction whose victim has already expired counts as an expiration, not an eviction, so a TTL cache under capacity pressure reports fewer evictions than the inserts that displaced an entry.
set_multi, get_multi, remove_multi, get_with_ttl, stats, path, sync, unlink, freeze, frozen, readonly and memfd are method-only (no keyword form).
Keywords take their arguments as a list, so a keyword that takes more than one argument must be written without parentheses around them:
shm_ii_put $map, $key, $value; # correct
shm_ii_put($map, $key, $value); # error, usually at compile timeA single-argument keyword accepts either form. The method call $map->put($key, $value) is always available if you prefer parentheses.
keys, values, items, each, get_multi, get_with_ttl, pop, shift, drain, flush_expired_partial and the cursor's next return lists. Like any Perl sub returning a list, in scalar context they yield their last element -- not a count, and not the first -- so call them in list context and use size when you want a count.
Calling no Data::HashMap::Shared::II; disables that variant's keywords for the rest of the enclosing lexical scope.
File management:
$map->sync; # flush the mmap to the backing file (msync MS_SYNC)
$map->unlink; # remove backing file (mmap stays valid)
Data::HashMap::Shared::II->unlink($path); # class method form (single file)sync issues a synchronous msync(2) over the whole mapping (every shard, for sharded maps) and dies on error. Use it to force durability of a file-backed map; it is a no-op for anonymous mappings, which have no backing file. Changes are visible to other processes sharing the mapping without sync -- it only affects on-disk persistence.
unlink reports through its return value rather than by dying: it returns true when the file (every shard, for sharded maps) was removed and false otherwise, including when the file was already gone and when removal was refused -- a read-only directory, for instance. Check it if the removal mattered.
Frozen (Read-Only) Mode
$map->freeze; # seal the file immutable (durable)
my $ro = Data::HashMap::Shared::II->new_readonly($path);
my $v = $ro->get($key); # lock-free query; writes nothing
my $is_frozen = $map->frozen; # true once sealed
my $is_readonly = $ro->readonly; # true for a read-only handlefreeze permanently seals a map's contents so it can be shipped and served read-only (it works on anonymous and memfd maps too, though only a file can be shipped). It takes the write lock and flushes the sealed header to disk, so the seal is durable. Afterwards every mutator on that handle croaks and the handle itself becomes read-only. A sharded map seals every shard file. Freezing is one-way; there is no unfreeze.
Quiesce your writers first. A mutator tests the seal on entry and takes the write lock afterwards, so another process already inside a mutating call when freeze runs completes its write after the seal: the sealed file changes once more, and a new_readonly reader can observe it. Seal a map only when nothing else is writing to it; freeze cannot detect a writer that has passed the check but not yet reached the lock.
On a sharded map this is not one straggling write. Whole-map and batch operations -- set_multi, remove_multi, clear, drain, pop, shift, flush_expired, flush_expired_partial, reserve -- test the seal once and then take each shard's lock in turn, so one that is under way when freeze lands keeps writing for the whole remainder of the call, across every shard it has not reached yet.
new_readonly opens an already-frozen file with O_RDONLY and maps it PROT_READ. Queries take no lock at all -- no reader-slot bookkeeping, no LRU clock bit, no lazy TTL cleanup -- and never write the mapping, so a read-only view works from a read-only file or filesystem, and any number of processes can share one frozen file at once. All queries and full iteration are supported: get, exists, get_with_ttl, get_multi, keys, values, items, to_hash, each, and cursors (cursor, cursor_next, cursor_reset and cursor_seek). Every mutator croaks, including the integer counters incr/decr/max/min. sync is a silent no-op. frozen and readonly report the state, and stats gains matching frozen and readonly keys.
The on-disk format and version are unchanged by the seal: a file written by an older release is simply not frozen and opens read-write exactly as before.
The two modes never mix: opening a frozen file read-write (new, new_from_fd) is refused -- open it with new_readonly instead -- and new_readonly refuses a file that has not been frozen.
new_readonly is for a single backing file, and there is no read-only sharded constructor, so freeze on a sharded map seals a set that no constructor will reopen: new_sharded refuses the frozen shards, and reading it back means opening each shard file by name and probing them. Freeze single-file maps.
Portability: a frozen file is a raw memory image. Read it back on the same architecture that wrote it (same word size and endianness; the native magic and variant id reject a mismatched or wrong-variant file at attach time). Ship it by copying the file; do not serve it over NFS or another network filesystem while another host has it mapped.
Crash Safety
If a process dies (e.g., SIGKILL, OOM kill) while holding the write lock, other processes detect the stale lock within 2 seconds and automatically recover.
Reader-side recovery uses a 1024-slot table in the shared mmap (one slot per handle, claimed lazily on first lock -- a process holding several handles on one map uses a slot for each; fork()'d children claim a fresh slot via pthread_atfork). A dead reader is neutralised by a draining writer, which clears its slot as it scans, so a worker killed mid-incr_by cannot pin the lock. Beyond 1024 simultaneous handles per map, a handle that cannot claim a slot proceeds "slotless"; see "Reader-slot exhaustion" for the one case that recovery cannot cover.
The same path validates and rebuilds the LRU doubly-linked list if a dead writer left it inconsistent. stat_recoveries in stats counts stale write-lock recoveries; a dead reader drained by a writer is not counted, so the counter staying at zero does not mean nothing has been recovered.
Recovery uses kill($pid, 0) for liveness, which cannot tell a reused PID from the original -- and the lock word lives in the file, so it lasts as long as the file does. Within one running system the risk is small: the holder must die in the window it holds the lock and the kernel must reissue that exact PID to a long-lived process before the next waiter looks.
It is not small once the file outlives the PID space that wrote it. A reboot, a container restart against a persisted volume, or a copy taken while a writer held the lock leaves a lock word naming a PID the new system may already have reissued. If it went to a long-lived process, every writer waits on a holder that will never release, unbounded and silent: no error, no warning, no timeout; readers wait too when the crash was mid-publish. Nothing in the API can break such a lock -- the file has to be recreated. A killed reader strands its slot the same way, since that records a PID too, and the read lock is held by each, keys, values, items and, without LRU or TTL, by incr, max and min. So carry a map across a reboot or a container restart only if every process that used it exited cleanly, and copy one only while nothing is using it.
Limitation: PID-based recovery assumes all processes share the same PID namespace. Cross-container sharing (different PID namespaces) is not supported.
A full filesystem arrives as SIGBUS, not as an error. The backing file is sized once, at creation, for the map's maximum geometry, and its table and arena are otherwise sparse: growing the table writes into pages that were never allocated rather than extending the file. So mmap_size is the space the file will need once every page has been touched, not what it occupies now, and if the filesystem fills while a page is first written, the kernel raises SIGBUS in the writing process instead of returning an error. One raised in the middle of a table resize takes with it the entries not yet re-inserted, exactly as a SIGKILL there would. Leave mmap_size bytes of headroom on the filesystem, or fallocate -l the file after creating it to take the allocation failure up front rather than at an arbitrary later insert. Use exactly mmap_size bytes: a file longer than the size recorded in its header is refused as corrupt, and for a sharded set mmap_size is the total across shards, so use each shard file's own size rather than the aggregate.
After recovery from a mid-mutation crash, the map data may be partially inconsistent (e.g., one entry was being updated when the writer died). Locks, the LRU chain and the entry counters are restored. The arena free lists are not rebuilt, so blocks in flight at the crash may leak; the specific entry being mutated may have stale or partial bytes; and a crash part-way through a table resize permanently drops the entries that had not yet been re-inserted. Calling clear after detecting a stale lock recovery is recommended for safety-critical applications.
An interrupted create is recovered too. A creator killed after the file is sized but before its header is committed leaves a full-size, all-zero file, which new re-initializes -- but only when the file is exactly the size the requested geometry needs, is owned by your effective uid, and is still entirely zero. A file holding data is never re-initialized. Once the first header field has landed the file can no longer be told from a corrupt one, and new croaks with incomplete map file left by an interrupted create; remove it and retry. An abandoned create never held data, so removing it is safe -- but a header corrupted after the fact reaches the same croak, so check before deleting anything you care about.
Recovery is run by whichever process next takes a lock, readers included, so a map shared with a process running anything older than 0.18 keeps that release's crash windows. Upgrade every process sharing a map together.
Reader-slot exhaustion
A reader that cannot claim a slot in the table described under "Crash Safety" proceeds "slotless": it still takes the read lock but leaves no per-process record, so if it is killed while holding the lock its share cannot be attributed to a dead process. Writer recovery cannot reclaim it and writers may block until the mapping is recreated. Reaching this needs more than 1024 handles open on one mapping at once plus a crash in the brief read-lock window, so in practice it is very unlikely.
BENCHMARKS
Throughput versus other shared-memory / on-disk solutions, 25K entries, single process, Linux x86_64. Each benchmarked sub runs over all 25,000 entries, so the figures below are Benchmark rates -- whole passes per second, higher is better -- and not operations per second. Multiply by 25,000 for the rate of the operation named: Shared::II LOOKUP at 353 is about 8.8 M lookups/s. (A pass can do more than the operation it is named for: DELETE refills the map first.) The cross-process table further down is already in operations per second. Run perl -Mblib bench/vs.pl 25000 to reproduce.
Integer key - integer value> (Shared::II):
BerkeleyDB LMDB Shared::II
INSERT 30 44 280
LOOKUP 38 38 353
INCREMENT 16 17 247String key - string value, short> (inline <= 7B, Shared::SS):
FastMmap BerkeleyDB LMDB SharedMem Shared::SS
INSERT 17 30 43 64 189
LOOKUP 15 35 36 154 220
DELETE -- 15 19 34 101String key - string value, long> (~50-100B, Shared::SS):
BerkeleyDB LMDB SharedMem Shared::SS
INSERT 26 40 63 196
LOOKUP 34 35 136 248LRU cache lookup (25K entries, lock-free clock eviction):
plain LRU
II 342 327 (lock-free; within run-to-run noise of plain)
SS 165 164Cross-process (25K SS entries, 2 processes, ops/s):
Shared::SS SharedMem LMDB
READS 3,798,000 3,085,000 854,000
WRITES 2,424,000 984,000 130,000
MIXED 50/50 5,738,000 2,470,000 275,000LMDB benchmarked with MDB_WRITEMAP|MDB_NOSYNC|MDB_NOMETASYNC|MDB_NORDAHEAD. BerkeleyDB with DB_PRIVATE|128MB cache.
Key takeaways:
9x faster lookups than LMDB for integer keys (lock-free seqlock path)
1.4x faster than Hash::SharedMem for short string lookups (inline strings, no arena overhead)
1.8x faster than Hash::SharedMem for long string lookups
4.4x faster cross-process reads than LMDB; 2.5x faster writes than SharedMem
LRU reads are lock-free (clock eviction) -- no overhead vs plain maps
Atomic
incris 14x faster than get+put on competitorsStrings <= 7 bytes stored inline in node (zero arena overhead)
SEE ALSO
Data::HashMap::Shared::Cookbook - recipes for counters, caches, rate limits, liveness, dedup and atomic state
Data::Buffer::Shared - typed shared array
Data::Queue::Shared - FIFO queue
Data::PubSub::Shared - publish-subscribe ring
Data::ReqRep::Shared - request-reply
Data::Sync::Shared - synchronization primitives
Data::Pool::Shared - fixed-size object pool
Data::Stack::Shared - LIFO stack
Data::Deque::Shared - double-ended queue
Data::Log::Shared - append-only log (WAL)
Data::Heap::Shared - priority queue
Data::Graph::Shared - directed weighted graph
Data::BitSet::Shared - shared bitset (lock-free per-bit ops)
Data::RingBuffer::Shared - fixed-size overwriting ring buffer
SECURITY
Backing files are created with mode 0600 (owner-only) by default, so only the creating user can open and attach them. To share a backing file across users, pass an explicit octal file mode such as 0660 as the last argument to new; the mode is applied when the file is created, and when a file left behind by an interrupted create is re-initialized (see "Crash Safety"); a file already in use keeps its own permissions. The file is opened with O_NOFOLLOW, so a symlink planted at the path is refused, and created with O_EXCL; the on-disk header is validated when the file is attached. Any process you grant write access to a shared mapping is trusted not to corrupt its contents while other processes are using it.
Header validation does not extend to the entries themselves. A string offset or length that falls outside the arena yields an empty string, a zeroed value or no match rather than a read past the mapping, on the write-locked paths as much as on the lock-free reads. Only those arena bounds are enforced. The rest of a map's per-slot data -- the LRU links above all -- is trusted, so behaviour on a corrupted file is undefined: it may return wrong answers, crash, or write outside the mapping. Corruption is out of the threat model, not defended against.
A backing file written before 0.16 uses the previous on-disk format and is rejected with a version-mismatch error when attached; recreate the map from its source data. Anonymous and memfd maps are process-local and unaffected.
AUTHOR
vividsnow
LICENSE
This is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
It bundles xxHash by Yann Collet, used under the BSD 2-Clause licence; see LICENSE.xxhash in the distribution.