Provided by: libcache-fastmmap-perl_1.46-1build2_amd64 
NAME
Cache::FastMmap - Uses an mmap'ed file to act as a shared memory interprocess cache
SYNOPSIS
use Cache::FastMmap;
# Uses vaguely sane defaults
$Cache = Cache::FastMmap->new();
# Uses Storable to serialize $Value to bytes for storage
$Cache->set($Key, $Value);
$Value = $Cache->get($Key);
$Cache = Cache::FastMmap->new(serializer => '');
# Stores stringified bytes of $Value directly
$Cache->set($Key, $Value);
$Value = $Cache->get($Key);
ABSTRACT
A shared memory cache through an mmap'ed file. It's core is written in C for performance.
It uses fcntl locking to ensure multiple processes can safely access the cache at the same
time. It uses a basic LRU algorithm to keep the most used entries in the cache.
DESCRIPTION
In multi-process environments (eg mod_perl, forking daemons, etc), it's common to want to
cache information, but have that cache shared between processes. Many solutions already
exist, and may suit your situation better:
• MLDBM::Sync - acts as a database, data is not automatically expired, slow
• IPC::MM - hash implementation is broken, data is not automatically expired, slow
• Cache::FileCache - lots of features, slow
• Cache::SharedMemoryCache - lots of features, VERY slow. Uses IPC::ShareLite which
freeze/thaws ALL data at each read/write
• DBI - use your favourite RDBMS. can perform well, need a DB server running. very
global. socket connection latency
• Cache::Mmap - similar to this module, in pure perl. slows down with larger pages
• BerkeleyDB - very fast (data ends up mostly in shared memory cache) but acts as a
database overall, so data is not automatically expired
In the case I was working on, I needed:
• Automatic expiry and space management
• Very fast access to lots of small items
• The ability to fetch/store many items in one go
Which is why I developed this module. It tries to be quite efficient through a number of
means:
• Core code is written in C for performance
• It uses multiple pages within a file, and uses Fcntl to only lock a page at a time to
reduce contention when multiple processes access the cache.
• It uses a dual level hashing system (hash to find page, then hash within each page to
find a slot) to make most "get()" calls O(1) and fast
• On each "set()", if there are slots and page space available, only the slot has to be
updated and the data written at the end of the used data space. If either runs out, a
re-organisation of the page is performed to create new slots/space which is done in an
efficient way
The class also supports read-through, and write-back or write-through callbacks to access
the real data if it's not in the cache, meaning that code like this:
my $Value = $Cache->get($Key);
if (!defined $Value) {
$Value = $RealDataSource->get($Key);
$Cache->set($Key, $Value)
}
Isn't required, you instead specify in the constructor:
Cache::FastMmap->new(
...
context => $RealDataSourceHandle,
read_cb => sub { $_[0]->get($_[1]) },
write_cb => sub { $_[0]->set($_[1], $_[2]) },
);
And then:
my $Value = $Cache->get($Key);
$Cache->set($Key, $NewValue);
Will just work and will be read/written to the underlying data source as needed
automatically.
PERFORMANCE
If you're storing relatively large and complex structures into the cache, then you're
limited by the speed of the Storable module. If you're storing simple structures, or raw
data, then Cache::FastMmap has noticeable performance improvements.
See <http://cpan.robm.fastmail.fm/cache_perf.html> for some comparisons to other modules.
COMPATIBILITY
Cache::FastMmap uses mmap to map a file as the shared cache space, and fcntl to do page
locking. This means it should work on most UNIX like operating systems.
Ash Berlin has written a Win32 layer using MapViewOfFile et al. to provide support for
Win32 platform.
MEMORY SIZE
Because Cache::FastMmap mmap's a shared file into your processes memory space, this can
make each process look quite large, even though it's just mmap'd memory that's shared
between all processes that use the cache, and may even be swapped out if the cache is
getting low usage.
However, the OS will think your process is quite large, which might mean you hit some
BSD::Resource or 'ulimits' you set previously that you thought were sane, but aren't
anymore, so be aware.
CACHE FILES AND OS ISSUES
Because Cache::FastMmap uses an mmap'ed file, when you put values into the cache, you are
actually "dirtying" pages in memory that belong to the cache file. Your OS will want to
write those dirty pages back to the file on the actual physical disk, but the rate it does
that at is very OS dependent.
In Linux, you have some control over how the OS writes those pages back using a number of
parameters in /proc/sys/vm
dirty_background_ratio
dirty_expire_centisecs
dirty_ratio
dirty_writeback_centisecs
How you tune these depends heavily on your setup.
As an interesting point, if you use a highmem linux kernel, a change between 2.6.16 and
2.6.20 made the kernel flush memory a LOT more. There's details in this kernel mailing
list thread: <http://www.uwsg.iu.edu/hypermail/linux/kernel/0711.3/0804.html>
In most cases, people are not actually concerned about the persistence of data in the
cache, and so are happy to disable writing of any cache data back to disk at all. Baically
what they want is an in memory only shared cache. The best way to do that is to use a
"tmpfs" filesystem and put all cache files on there.
For instance, all our machines have a /tmpfs mount point that we create in /etc/fstab as:
none /tmpfs tmpfs defaults,noatime,size=1000M 0 0
And we put all our cache files on there. The tmpfs filesystem is smart enough to only use
memory as required by files actually on the tmpfs, so making it 1G in size doesn't
actually use 1G of memory, it only uses as much as the cache files we put on it. In all
cases, we ensure that we never run out of real memory, so the cache files effectively act
just as named access points to shared memory.
Some people have suggested using anonymous mmaped memory. Unfortunately we need a file
descriptor to do the fcntl locking on, so we'd have to create a separate file on a
filesystem somewhere anyway. It seems easier to just create an explicit "tmpfs"
filesystem.
PAGE SIZE AND KEY/VALUE LIMITS
To reduce lock contention, Cache::FastMmap breaks up the file into pages. When you get/set
a value, it hashes the key to get a page, then locks that page, and uses a hash table
within the page to get/store the actual key/value pair.
One consequence of this is that you cannot store values larger than a page in the cache at
all. Attempting to store values larger than a page size will fail (the set() function will
return false).
Also keep in mind that each page has it's own hash table, and that we store the key and
value data of each item. So if you are expecting to store large values and/or keys in the
cache, you should use page sizes that are definitely larger than your largest key + value
size + a few kbytes for the overhead.
USAGE
Because the cache uses shared memory through an mmap'd file, you have to make sure each
process connects up to the file. There's probably two main ways to do this:
• Create the cache in the parent process, and then when it forks, each child will
inherit the same file descriptor, mmap'ed memory, etc and just work. This is the
recommended way. (BEWARE: This only works under UNIX as Win32 has no concept of
forking)
• Explicitly connect up in each forked child to the share file. In this case, make sure
the file already exists and the children connect with init_file => 0 to avoid deleting
the cache contents and possible race corruption conditions. Also be careful that
multiple children may race to create the file at the same time, each overwriting and
corrupting content. Use a separate lock file if you must to ensure only one child
creates the file. (This is the only possible way under Win32)
The first way is usually the easiest. If you're using the cache in a Net::Server based
module, you'll want to open the cache in the "pre_loop_hook", because that's executed
before the fork, but after the process ownership has changed and any chroot has been done.
In mod_perl, just open the cache at the global level in the appropriate module, which is
executed as the server is starting and before it starts forking children, but you'll
probably want to chmod or chown the file to the permissions of the apache process.
METHODS
new(%Opts)
Create a new Cache::FastMmap object.
Basic global parameters are:
• share_file
File to mmap for sharing of data. default on unix:
/tmp/sharefile-$pid-$time-$random default on windows:
%TEMP%\sharefile-$pid-$time-$random
• init_file
Clear any existing values and re-initialise file. Useful to do in a parent that
forks off children to ensure that file is empty at the start (default: 0)
Note: This is quite important to do in the parent to ensure a consistent file
structure. The shared file is not perfectly transaction safe, and so if a child is
killed at the wrong instant, it might leave the cache file in an inconsistent
state.
• serializer
Use a serialization library to serialize perl data structures before storing in
the cache. If not set, the raw value in the variable passed to set() is stored as
a string. You must set this if you want to store anything other than basic scalar
values. Supported values are:
'' for none
'storable' for 'Storable'
'sereal' for 'Sereal'
'json' for 'JSON'
If this parameter has a value the module will attempt to load the associated
package and then use the API of that package to serialize data before storing in
the cache, and deserialize it upon retrieval from the cache. (default: 'storable')
(Note: Historically this module only supported a boolean value for the
`raw_values` parameter and defaulted to 0, which meant it used Storable to
serialze all values.)
• raw_values
Deprecated. Use serializer above
• compressor
Compress the value (but not the key) before storing into the cache, using the
compression package identified by the value of the parameter. Supported values
are:
'zlib' for 'Compress::Zlib'
'lz4' for 'Compress::LZ4'
'snappy' for 'Compress::Snappy'
If this parameter has a value the module will attempt to load the associated
package and then use the API of that package to compress data before storing in
the cache, and uncompress it upon retrieval from the cache. (default: undef)
(Note: Historically this module only supported a boolean value for the `compress`
parameter and defaulted to use Compress::Zlib. The note for the old `compress`
parameter stated: "Some initial testing shows that the uncompressing tends to be
very fast, though the compressing can be quite slow, so it's probably best to use
this option only if you know values in the cache are long-lived and have a high
hit rate."
Comparable test results for the other compression tools are not yet available;
submission of benchmarks welcome. However, the documentation for the 'Snappy'
library (http://google.github.io/snappy/) states: For instance, compared to the
fastest mode of zlib, Snappy is an order of magnitude faster for most inputs, but
the resulting compressed files are anywhere from 20% to 100% bigger. )
• compress
Deprecated. Please use compressor, see above.
• enable_stats
Enable some basic statistics capturing. When enabled, every read to the cache is
counted, and every read to the cache that finds a value in the cache is also
counted. You can then retrieve these values via the get_statistics() call. This
causes every read action to do a write on a page, which can cause some more IO, so
it's disabled by default. (default: 0)
• expire_time
Maximum time to hold values in the cache in seconds. A value of 0 means does no
explicit expiry time, and values are expired only based on LRU usage. Can be
expressed as 1m, 1h, 1d for minutes/hours/days respectively. (default: 0)
You may specify the cache size as:
• cache_size
Size of cache. Can be expresses as 1k, 1m for kilobytes or megabytes respectively.
Automatically guesses page size/page count values.
Or specify explicit page size/page count values. If none of these are specified, the
values page_size = 64k and num_pages = 89 are used.
• page_size
Size of each page. Must be a power of 2 between 4k and 1024k. If not, is rounded
to the nearest value.
• num_pages
Number of pages. Should be a prime number for best hashing
The cache allows the use of callbacks for reading/writing data to an underlying data
store.
• context
Opaque reference passed as the first parameter to any callback function if
specified
• read_cb
Callback to read data from the underlying data store. Called as:
$read_cb->($context, $Key)
Should return the value to use. This value will be saved in the cache for future
retrievals. Return undef if there is no value for the given key
• write_cb
Callback to write data to the underlying data store. Called as:
$write_cb->($context, $Key, $Value, $ExpiryTime)
In 'write_through' mode, it's always called as soon as a set(...) is called on
the Cache::FastMmap class. In 'write_back' mode, it's called when a value is
expunged from the cache if it's been changed by a set(...) rather than read from
the underlying store with the read_cb above.
Note: Expired items do result in the write_cb being called if 'write_back' caching
is enabled and the item has been changed. You can check the $ExpiryTime against
"time()" if you only want to write back values which aren't expired.
Also remember that write_cb may be called in a different process to the one that
placed the data in the cache in the first place
• delete_cb
Callback to delete data from the underlying data store. Called as:
$delete_cb->($context, $Key)
Called as soon as remove(...) is called on the Cache::FastMmap class
• cache_not_found
If set to true, then if the read_cb is called and it returns undef to say nothing
was found, then that information is stored in the cache, so that next time a
get(...) is called on that key, undef is returned immediately rather than again
calling the read_cb
• write_action
Either 'write_back' or 'write_through'. (default: write_through)
• allow_recursive
If you're using a callback function, then normally the cache is not re-enterable,
and attempting to call a get/set on the cache will cause an error. By setting this
to one, the cache will unlock any pages before calling the callback. During the
unlock time, other processes may change data in current cache page, causing
possible unexpected effects. You shouldn't set this unless you know you want to be
able to recall to the cache within a callback. (default: 0)
• empty_on_exit
When you have 'write_back' mode enabled, then you really want to make sure all
values from the cache are expunged when your program exits so any changes are
written back.
The trick is that we only want to do this in the parent process, we don't want any
child processes to empty the cache when they exit. So if you set this, it takes
the PID via $$, and only calls empty in the DESTROY method if $$ matches the pid
we captured at the start. (default: 0)
• unlink_on_exit
Unlink the share file when the cache is destroyed.
As with empty_on_exit, this will only unlink the file if the DESTROY occurs in the
same PID that the cache was created in so that any forked children don't unlink
the file.
This value defaults to 1 if the share_file specified does not already exist. If
the share_file specified does already exist, it defaults to 0.
• catch_deadlocks
Sets an alarm(10) before each page is locked via fcntl(F_SETLKW) to catch any
deadlock. This used to be the default behaviour, but it's not really needed in the
default case and could clobber sub-second Time::HiRes alarms setup by other code.
Defaults to 0.
get($Key, [ \%Options ])
Search cache for given Key. Returns undef if not found. If read_cb specified and not
found, calls the callback to try and find the value for the key, and if found (or
'cache_not_found' is set), stores it into the cache and returns the found value.
%Options is optional, and is used by get_and_set() to control the locking behaviour.
For now, you should probably ignore it unless you read the code to understand how it
works
set($Key, $Value, [ \%Options ])
Store specified key/value pair into cache
%Options is optional, and is used by get_and_set() to control the locking behaviour.
For now, you should probably ignore it unless you read the code to understand how it
works
This method returns true if the value was stored in the cache, false otherwise. See
the PAGE SIZE AND KEY/VALUE LIMITS section for more details.
get_and_set($Key, $Sub)
Atomically retrieve and set the value of a Key.
The page is locked while retrieving the $Key and is unlocked only after the value is
set, thus guaranteeing the value does not change between the get and set operations.
$Sub is a reference to a subroutine that is called to calculate the new value to
store. $Sub gets $Key and the current value as parameters, and should return the new
value to set in the cache for the given $Key.
If the subroutine returns an empty list, no value is stored back in the cache. This
avoids updating the expiry time on an entry if you want to do a "get if in cache,
store if not present" type callback.
For example, to atomically increment a value in the cache, you can just use:
$Cache->get_and_set($Key, sub { return ++$_[1]; });
In scalar context, the return value from this function is the *new* value stored back
into the cache.
In list context, a two item array is returned; the new value stored back into the
cache and a boolean that's true if the value was stored in the cache, false otherwise.
See the PAGE SIZE AND KEY/VALUE LIMITS section for more details.
Notes:
• Do not perform any get/set operations from the callback sub, as these operations
lock the page and you may end up with a dead lock!
• If your sub does a die/throws an exception, the page will correctly be unlocked
(1.15 onwards)
remove($Key, [ \%Options ])
Delete the given key from the cache
%Options is optional, and is used by get_and_remove() to control the locking
behaviour. For now, you should probably ignore it unless you read the code to
understand how it works
get_and_remove($Key)
Atomically retrieve value of a Key while removing it from the cache.
The page is locked while retrieving the $Key and is unlocked only after the value is
removed, thus guaranteeing the value stored by someone else isn't removed by us.
clear()
Clear all items from the cache
Note: If you're using callbacks, this has no effect on items in the underlying data
store. No delete callbacks are made
purge()
Clear all expired items from the cache
Note: If you're using callbacks, this has no effect on items in the underlying data
store. No delete callbacks are made, and no write callbacks are made for the expired
data
empty($OnlyExpired)
Empty all items from the cache, or if $OnlyExpired is true, only expired items.
Note: If 'write_back' mode is enabled, any changed items are written back to the
underlying store. Expired items are written back to the underlying store as well.
get_keys($Mode)
Get a list of keys/values held in the cache. May immediately be out of date because of
the shared access nature of the cache
If $Mode == 0, an array of keys is returned
If $Mode == 1, then an array of hashrefs, with 'key', 'last_access', 'expire_time' and
'flags' keys is returned
If $Mode == 2, then hashrefs also contain 'value' key
get_statistics($Clear)
Returns a two value list of (nreads, nreadhits). This only works if you passed
enable_stats in the constructor
nreads is the total number of read attempts done on the cache since it was created
nreadhits is the total number of read attempts done on the cache since it was created
that found the key/value in the cache
If $Clear is true, the values are reset immediately after they are retrieved
multi_get($PageKey, [ $Key1, $Key2, ... ])
The two multi_xxx routines act a bit differently to the other routines. With the
multi_get, you pass a separate PageKey value and then multiple keys. The PageKey value
is hashed, and that page locked. Then that page is searched for each key. It returns a
hash ref of Key => Value items found in that page in the cache.
The main advantage of this is just a speed one, if you happen to need to search for a
lot of items on each call.
For instance, say you have users and a bunch of pieces of separate information for
each user. On a particular run, you need to retrieve a sub-set of that information for
a user. You could do lots of get() calls, or you could use the 'username' as the page
key, and just use one multi_get() and multi_set() call instead.
A couple of things to note:
1. This makes multi_get()/multi_set() and get()/set() incompatible. Don't mix calls
to the two, because you won't find the data you're expecting
2. The writeback and callback modes of operation do not work with
multi_get()/multi_set(). Don't attempt to use them together.
multi_set($PageKey, { $Key1 = $Value1, $Key2 => $Value2, ... }, [ \%Options ])>
Store specified key/value pair into cache
INTERNAL METHODS
_expunge_all($Mode, $WB)
Expunge all items from the cache
Expunged items (that have not expired) are written back to the underlying store if
write_back is enabled
_expunge_page($Mode, $WB, $Len)
Expunge items from the current page to make space for $Len bytes key/value items
Expunged items (that have not expired) are written back to the underlying store if
write_back is enabled
_lock_page($Page)
Lock a given page in the cache, and return an object reference that when DESTROYed,
unlocks the page
INCOMPATIBLE CHANGES
• From 1.15
• Default share_file name is no-longer /tmp/sharefile, but
/tmp/sharefile-$pid-$time. This ensures that different runs/processes don't
interfere with each other, but means you may not connect up to the file you
expect. You should be choosing an explicit name in most cases.
On Unix systems, you can pass in the environment variable TMPDIR to override the
default directory of /tmp
• The new option unlink_on_exit defaults to true if you pass a filename for the
share_file which doesn't already exist. This means if you have one process that
creates the file, and another that expects the file to be there, by default it
won't be.
Otherwise the defaults seem sensible to cleanup unneeded share files rather than
leaving them around to accumulate.
• From 1.29
• Default share_file name is no longer /tmp/sharefile-$pid-$time but
/tmp/sharefile-$pid-$time-$random.
• From 1.31
• Before 1.31, if you were using raw_values => 0 mode, then the write_cb would be
called with raw frozen data, rather than the thawed object. From 1.31 onwards, it
correctly calls write_cb with the thawed object value (eg what was passed to the
->set() call in the first place)
• From 1.36
• Before 1.36, an alarm(10) would be set before each attempt to lock a page. The
only purpose of this was to detect deadlocks, which should only happen if the
Cache::FastMmap code was buggy, or a callback function in get_and_set() made
another call into Cache::FastMmap.
However this added unnecessary extra system calls for every lookup, and for users
using Time::HiRes, it could clobber any existing alarms that had been set with
sub-second resolution.
So this has now been made an optional feature via the catch_deadlocks option
passed to new.
SEE ALSO
MLDBM::Sync, IPC::MM, Cache::FileCache, Cache::SharedMemoryCache, DBI, Cache::Mmap,
BerkeleyDB
Latest news/details can also be found at:
<http://cpan.robm.fastmail.fm/cachefastmmap/>
Available on github at:
<https://github.com/robmueller/cache-fastmmap/>
AUTHOR
Rob Mueller <mailto:cpan@robm.fastmail.fm>
COPYRIGHT AND LICENSE
Copyright (C) 2003-2017 by FastMail Pty Ltd
This library is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.