On Wed, Jan 09, 2019 at 09:44:28AM -0800, Shakeel Butt wrote: > Hi Johannes, > > On Wed, Jan 9, 2019 at 8:45 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > > > On Wed, Jan 09, 2019 at 03:20:18PM +0300, Kirill Tkhai wrote: > > > On nodes without memory overcommit, it's common a situation, > > > when memcg exceeds its limit and pages from pagecache are > > > shrinked on reclaim, while node has a lot of free memory. > > > Further access to the pages requires real device IO, while > > > IO causes time delays, worse powerusage, worse throughput > > > for other users of the device, etc. > > > > > > Cleancache is not a good solution for this problem, since > > > it implies copying of page on every cleancache_put_page() > > > and cleancache_get_page(). Also, it requires introduction > > > of internal per-cleancache_ops data structures to manage > > > cached pages and their inodes relationships, which again > > > introduces overhead. > > > > > > This patchset introduces another solution. It introduces > > > a new scheme for evicting memcg pages: > > > > > > 1)__remove_mapping() uncharges unmapped page memcg > > > and leaves page in pagecache on memcg reclaim; > > > > > > 2)putback_lru_page() places page into root_mem_cgroup > > > list, since its memcg is NULL. Page may be evicted > > > on global reclaim (and this will be easily, as > > > page is not mapped, so shrinker will shrink it > > > with 100% probability of success); > > > > > > 3)pagecache_get_page() charges page into memcg of > > > a task, which takes it first. > > > > > > Below is small test, which shows profit of the patchset. > > > > > > Create memcg with limit 20M (exact value does not matter much): > > > $ mkdir /sys/fs/cgroup/memory/ct > > > $ echo 20M > /sys/fs/cgroup/memory/ct/memory.limit_in_bytes > > > $ echo $$ > /sys/fs/cgroup/memory/ct/tasks > > > > > > Then twice read 1GB file: > > > $ time cat file_1gb > /dev/null > > > > > > Before (2 iterations): > > > 1)0.01user 0.82system 0:11.16elapsed 7%CPU > > > 2)0.01user 0.91system 0:11.16elapsed 8%CPU > > > > > > After (2 iterations): > > > 1)0.01user 0.57system 0:11.31elapsed 5%CPU > > > 2)0.00user 0.28system 0:00.28elapsed 100%CPU > > > > > > With the patch set applied, we have file pages are cached > > > during the second read, so the result is 39 times faster. > > > > > > This may be useful for slow disks, NFS, nodes without > > > overcommit by memory, in case of two memcg access the same > > > files, etc. > > > > What you're implementing is work conservation: avoid causing IO work, > > unless it's physically necessary, not when the memcg limit says so. > > > > This is a great idea, but we already have that in the form of the > > memory.low setting (or softlimit in cgroup v1). > > > > Say you have a 100M system and two cgroups. Instead of setting the 20M > > limit on group A as you did, you set 80M memory.low on group B. If B > > is not using its share and there is no physical memory pressure, group > > A can consume as much memory as it wants. If B starts and consumes its > > 80M, A will get pushed back to 20M. (And when B grows beyond 80M, they > > compete fairly over the remaining 20M, just like they would if A had > > the 20M limit setting). > > There is one difference between the example you give and the proposal. > In your example when B starts and consumes its 80M and pushes back A > to 20M, the direct reclaim can be very expensive and > non-deterministic. While in the proposal, the B's direct reclaim will > be very fast and deterministic (assuming no overcommit on hard limits) > as it will always first reclaim unmapped clean pages which were > charged to A. That struck me more as a side-effect of the implementation having to unmap the pages to be able to change their page->mem_cgroup. But regardless, we cannot fundamentally change the memory isolation semantics of the hard limit like these patches propose, so it's a moot point. A scheme to prepare likely reclaim candidates in advance for a low-latency workload startup would have to come in a different form.
