On 11/06/2012 01:48 AM, Andrew Morton wrote: > On Thu, 1 Nov 2012 16:07:41 +0400 > Glauber Costa <glommer@xxxxxxxxxxxxx> wrote: > >> This means that when we destroy a memcg cache that happened to be empty, >> those caches may take a lot of time to go away: removing the memcg >> reference won't destroy them - because there are pending references, and >> the empty pages will stay there, until a shrinker is called upon for any >> reason. >> >> In this patch, we will call kmem_cache_shrink for all dead caches that >> cannot be destroyed because of remaining pages. After shrinking, it is >> possible that it could be freed. If this is not the case, we'll schedule >> a lazy worker to keep trying. > > This patch is really quite nasty. We poll the cache once per minute > trying to shrink then free it? a) it gives rise to concerns that there > will be scenarios where the system could suffer unlimited memory windup > but mainly b) it's just lame. > > The kernel doesn't do this sort of thing. The kernel tries to be > precise: in a situation like this we keep track of the number of > outstanding objects and when that falls to zero, we free their > container synchronously. If those objects are normally left floating > around in an allocated but reclaimable state then we can address that > by synchronously freeing them if their container has been destroyed. > > Or something like that. If it's something else then fine, but not this. > > What do we need to do to fix this? > The original patch had a unlikely() test in the free path, conditional on whether or not the cache is dead, that would then call this is the cache would now be empty. I got several requests to remove it and change it to something like this, because that is a fast path (I myself think an unlikely branch is not that bad) If you think such a test is acceptable, I can bring it back and argue in the basis of "akpm made me do it!". But meanwhile I will give this extra though to see if there is any alternative way I can do it... -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>