Req #73887 [Com]: Improve realpath cache eviction strategy
Edit report at https://bugs.php.net/bug.php?id=73887&edit=1
ID: 73887
Comment by: spam2 at rhsoft dot net
Reported by: maggus dot staab at gmail dot com
Summary: Improve realpath cache eviction strategy
Status: Open
Type: Feature/Change Request
Package: Performance problem
PHP Version: any
Block user comment: N
Private report: N
New Comment:
someone pointed out offlist that this maybe is not clear:
put a file on a large and busy server with that line
<?php echo count(realpath_cache_get());?>
call that URL 4 times and you get that numbers below
325
6
57
1692
you can hardly tell me to get a useful hitrate with such randomness all over workers
Previous Comments:
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[2017-07-26 10:56:10] spam2 at rhsoft dot net
which performance do we really have?
please read the bugreport i linked
when i make 4 calls to a website and get that numbers below it's all said about the hitrate in
real life and how high it could be - it needs a lot of requests to fill all that cache instances and
after they are filled 'MaxRequestsPerChild' comes and kills the process, starts a new one
and it's cache is empty again - so what *real world* performance are we talking about?
clearstatcache(true, $path) is completly worthless because it resets the cache on one of propably
many hundret workers which makes the whole cache dangerous while without that issue you could set
"realpath_cache_ttl" to hours instead short values
with 4 MB realpath_cache_size you have probably 1000*4 MB in case of 1000 workers, watsing 4 GB RAM,
starting oom-killer or crashing the whole machine instead any benefit - so in summary the probably
needed locks are the smallest problem at all
echo count(realpath_cache_get());
325
6
57
1692
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[2017-07-26 10:26:43] maggus dot staab at gmail dot com
adding a shared cache would also imply a locking/concurrency mechanism which could also be less
performant and more complex in comparision what we have today.
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[2017-07-26 10:17:54] spam2 at rhsoft dot net
first it would be much more important to have the cache shared between workers and so fix the
split-brain and overhead of maintaining a cache for every process with a very bad hitrate on mahines
with hundrets of workers
see https://bugs.php.net/bug.php?id=73888 -
"We recently changed the default realpath cache" is a big mistake as long as verey single
worker has it's own cache with that size and clearstatcache() after change a file don't
work at all with the current design
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[2017-01-07 13:45:40] maggus dot staab at gmail dot com
Description:
------------
We recently changed the default realpath cache size as we realized that the default size is too
small, see https://github.com/php/php-src/pull/2271.
After a closer look into the realpath cache impl krakjoe and bwoebi mentioned, that cache eviction
of this cache is very suboptiomal. Cache items are only evicted after a find. Entries never get
evicted when no longer used.
Php is forced todo some expensive file-io operations when the realpath cache cannot deliver the
information from memory. This is especially expensive for long running php scripts, like cli based
processes/webservers/...
A first improvement would be some kind garbage collection, which isnt bound to a previous find.
2nd the cache could benefit from some kind of LRU cache.
3rd krakjoe mentioned that it might be sometimes usefull to share the cache information in shm.
Related discussion: http://chat.stackoverflow.com/transcript/message/34952738#34952738
Starting 5:32 PM
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