NAME
Data::HashMap::Shared - Type-specialized shared-memory hash maps for multiprocess access
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 (lock-free fast path)
shm_ii_incr $map, 1;
shm_ii_incr_by $map, 1, 10;
# 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.
Linux-only. Requires 64-bit Perl.
Features
File-backed mmap for cross-process sharing
Futex-based read-write lock (fast userspace path)
Lock-free atomic counters (incr/decr under read lock)
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 and per-key TTL (zero cost when disabled)
Stale lock recovery (automatic detection of dead lock holders via PID tracking)
Variants
Constructor
my $map = Data::HashMap::Shared::II->new($path, $max_entries);
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);Creates or opens a shared hash map backed by file $path. $max_entries, $max_size, and $ttl 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. 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. When LRU is active, get promotes the accessed entry, so reads take a write lock instead of the lock-free seqlock path.
Optional $ttl sets a default time-to-live in seconds for all entries. Expired entries are lazily removed on access. Set to 0 (default) to disable. When TTL is active, get and exists check expiry.
Zero-cost when disabled: with both $max_size=0 and $ttl=0, the fast lock-free read path is used. The only overhead is a branch (predicted away).
API
Replace xx with variant prefix: i16, i32, ii, i16s, i32s, is, si16, si32, si, ss.
my $ok = shm_xx_put $map, $key, $value; # false if table/arena full
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 valueInteger-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;LRU/TTL operations (require TTL-enabled map for put_ttl):
my $ok = shm_xx_put_ttl $map, $key, $value, $ttl_sec; # per-key TTL (0 = permanent); requires TTL-enabled map
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; # seconds left (0 = permanent, undef if missing/expired/no TTL)
my $ok = shm_xx_touch $map, $key; # reset TTL to default_ttl (no-op on permanent entries), promote LRU
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 slotsAtomic remove-and-return:
my $v = shm_xx_take $map, $key; # remove key and return value (undef if missing)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
shm_xx_cursor_seek $cur, $key; # position at specific key (best-effort across resize)
# cursor auto-destroyed when out of scopeshm_xx_each is also safe to use with remove during iteration. Resize/compaction is deferred until iteration ends.
Diagnostics:
my $cap = shm_xx_capacity $map; # current table capacity (slots)
my $tb = shm_xx_tombstones $map; # tombstone count
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)File management:
$map->unlink; # remove backing file (mmap stays valid)
Data::HashMap::Shared::II->unlink($path); # class method formCrash Safety
If a process dies (e.g., SIGKILL, OOM kill) while holding the write lock, other processes will detect the stale lock within 2 seconds via PID tracking and automatically recover. The writer's PID is encoded in the rwlock word itself (single atomic CAS, no crash window), so recovery is reliable even if the process is killed mid-acquisition. On timeout, waiters check kill($pid, 0) and CAS-release the lock if the holder is dead.
Limitation: PID-based recovery assumes all processes share the same PID namespace. Cross-container sharing (different PID namespaces) is not supported.
After recovery from a mid-mutation crash, the map data may be inconsistent. Calling clear after detecting a stale lock recovery is recommended for safety-critical applications.
BENCHMARKS
Throughput versus other shared-memory / on-disk solutions, 25K entries, single process, Linux x86_64. Run perl -Mblib bench/vs.pl 25000 to reproduce.
INTEGER KEY -> INTEGER VALUE (Shared::II)
Rate BerkeleyDB LMDB Shared::II
INSERT 30/s 30 42 172
LOOKUP 37/s 37 36 372
INCREMENT 15/s 15 17 164
STRING KEY -> STRING VALUE (Shared::SS)
Rate FastMmap BerkeleyDB LMDB SharedMem Shared::SS
INSERT 10/s 10 26 31 46 81
LOOKUP 10/s 10 33 30 102 151
DELETE 13/s 13 14 -- 27 45
CROSS-PROCESS (25K SS entries, 2 processes)
READS Shared::SS 2,594,000/s SharedMem 1,547,000/s LMDB 625,000/s
WRITES Shared::SS 2,211,000/s SharedMem 783,000/s LMDB 102,000/s
MIXED 50/50 Shared::SS 3,705,000/s SharedMem 1,981,000/s LMDB 226,000/sLMDB benchmarked with MDB_WRITEMAP|MDB_NOSYNC|MDB_NOMETASYNC|MDB_NORDAHEAD. BerkeleyDB with DB_PRIVATE|128MB cache.
Key takeaways:
10x faster lookups than LMDB for integer keys (lock-free seqlock path)
1.5x faster than Hash::SharedMem for string lookups
4x faster cross-process reads than LMDB; 3x faster writes than SharedMem
Atomic
incris 10x faster than get+put on competitors
AUTHOR
vividsnow
LICENSE
This is free software; you can redistribute it and/or modify it under the same terms as Perl itself.