| Project: | Memcache API and Integration |
| Version: | 5.x-1.9 |
| Component: | Code |
| Category: | bug report |
| Priority: | critical |
| Assigned: | Unassigned |
| Status: | closed (won't fix) |
Issue Summary
You can generate near-100% miss rates on certain caches in 1.9 if the entity being cached uses CACHE_TEMPORARY. This means that there is a > 2:1 WRITE TO READ ratio.
Here's the problem:
Notice that in the code below, $expire is not (re-)set until after its value is assigned to $cache->expire.
This seems to result in the behavior that every CACHE_TEMPORARY item expires every time. (See dmemcache_get() for fetching code -- -1 is definitely less than time()).
Furthermore, $expire is never set to the actual expiration time. It is just the number of seconds to wait. The expire time should be variable_get('cache_lifetime', 2591999) + $time
<?php
function cache_set($cid, $table = 'cache', $data, $expire = CACHE_PERMANENT, $headers = NULL) {
$time = time();
// Create new cache object.
$cache = new stdClass;
$cache->cid = $cid;
$cache->data = is_object($data) ? memcache_clone($data) : $data;
$cache->created = $time;
$cache->expire = $expire;
$cache->headers = $headers;
// Save to memcache
if ($expire == CACHE_TEMPORARY) {
$expire = variable_get('cache_lifetime', 2591999);
}
// We manually track the expire time in $cache->expire. When the object
// expires, we only allow one request to rebuild it to avoid cache stampedes.
// Other requests for the expired object while it is still being rebuilt get
// the expired object.
dmemcache_set($cid, $cache, 0, $table);
}
?>I believe this bug should be marked critical, given the fact that it actually hurts overall system performance, rather than helping.
Comments
#1
Patch attached.
#2
D'oh... sorry... should have marked this as "Needs Review"
#3
Will
#4
I have been looking at, what seems to be a similar issue (although mostly with cache clearing and D6):
http://drupal.org/node/572838#comment-2690370
#5
Just marked 5.x as unsupported, won't fixing this.