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Bug #58215 APC takes too long to free memory
Submitted: 2008-06-03 14:56 UTC Modified: 2008-08-12 10:49 UTC
From: gdiego at gmail dot com Assigned:
Status: Closed Package: APC (PECL)
PHP Version: 5.2.5 OS: Debian
Private report: No CVE-ID: None
 [2008-06-03 14:56 UTC] gdiego at gmail dot com
Description:
------------
When calling apc_store, if it needs memory, it might take a long time to free if several user variables are stored.

I think the problem comes with a modification at line 659 / 660 of php_apc.c that now has this code:

 apc_cache_expunge(apc_cache,t);
 apc_cache_expunge(apc_user_cache,t);

If several variables are stored in the cache then it takes a long time to free some.

Reproduce code:
---------------
<?
  set_time_limit(0);
  for ($i = 0; $i < 500000; $i++) {
    $u = apc_cache_info("user", true); $u = $u['mem_size'];
    if ($i % 50000 == 0) echo "--- $i $u\n";
    $x = microtime(true); apc_store($i, $i); $x = microtime(true) - $x;
    if ($x > 3) die("$x seconds taken for $i\n");
  }
  echo "test";
?>


Expected result:
----------------
An echo every time $i % 50000 == 0
test


Actual result:
--------------
--- 0 0
--- 50000 5177780
--- 100000 10377780
--- 150000 15677780
76.2400200367 seconds taken for 163626

(it takes a long time at iteration 163626 when cache is full)

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 [2008-06-26 08:11 UTC] gopalv82 at yahoo dot com
With apc-3.1.x CVS HEAD, with a slight change to your script

    $u = apc_cache_info("user", true); $e = $u['expunges']; $m = $u['mem_size'];
    if ($i % 50000 == 0) echo "--- $i $e $m\n";

--- 0 0 0
--- 50000 0 28000000
--- 100000 1 27072080
--- 150000 2 26144160
--- 200000 3 25216240
--- 250000 4 24288320
--- 300000 5 23360400
--- 350000 6 22432480
--- 400000 7 21504560
--- 450000 8 20576640

took 14 seconds with a 32Mb cache. Please test CVS HEAD of pecl/apc and update the bug
 [2008-06-26 09:18 UTC] gdiego at gmail dot com
Hi there,

even if it takes 14 seconds now, remember it's a php script (timeout: 30 seconds). What happens if you are using much more than 32MB (and a lot of variables)? Maybe the memory freeing process could be in another thread and run without locking all other operations.

Thanks for the reply!
 [2008-08-12 10:49 UTC] gopalv82 at yahoo dot com
Can't run it another thread or without locks.

Last benchmark put a complete 32Mb cache flush in 800ms, which should be sane enough.

More improvements on cache flushes coming up, but it's reached sane enough levels to close thsi bug
 
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