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();
# $Value must be a reference...
$Cache->set($Key, $Value);
$Value = $Cache->get($Key);
$Cache = Cache::FastMmap->new(raw_values => 1);
# $Value can't be a reference...
$Cache->set($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.
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
the cache file in an inconsistent state.
- •
- raw_values
Store values as raw binary data rather than using Storable to free/thaw data
structures (default: 0)
- •
- compress
Compress the value (but not the key) before storing into the cache. If you
set this to 1, the module will attempt to require the Compress::Zlib
module and then use the memGzip() function on the value data before
storing into the cache, and memGunzip() when retrieving data from
the cache. 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. (default: 0)
- •
- 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 betwen 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.
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/
<
https://github.com/robmueller/cache-fastmmap/>
AUTHOR¶
Rob Mueller <mailto:cpan@robm.fastmail.fm>
COPYRIGHT AND LICENSE¶
Copyright (C) 2003-2011 by Opera Software Australia Pty Ltd
This library is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.