On Wed, 7 Nov 2012 08:13:08 +0100 Glauber Costa <glommer@xxxxxxxxxxxxx> wrote: > 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... OK, thanks, please do take a look at it. I'd be interested in seeing the old version of the patch which had this test-n-branch. Perhaps there's some trick we can pull to lessen its cost. -- 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>