Messages in this thread | | | Date | Wed, 1 Mar 2017 12:36:28 -0500 | From | Johannes Weiner <> | Subject | Re: [PATCH 6/9] mm: don't avoid high-priority reclaim on memcg limit reclaim |
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On Wed, Mar 01, 2017 at 04:40:27PM +0100, Michal Hocko wrote: > On Tue 28-02-17 16:40:04, Johannes Weiner wrote: > > 246e87a93934 ("memcg: fix get_scan_count() for small targets") sought > > to avoid high reclaim priorities for memcg by forcing it to scan a > > minimum amount of pages when lru_pages >> priority yielded nothing. > > This was done at a time when reclaim decisions like dirty throttling > > were tied to the priority level. > > > > Nowadays, the only meaningful thing still tied to priority dropping > > below DEF_PRIORITY - 2 is gating whether laptop_mode=1 is generally > > allowed to write. But that is from an era where direct reclaim was > > still allowed to call ->writepage, and kswapd nowadays avoids writes > > until it's scanned every clean page in the system. Potential changes > > to how quick sc->may_writepage could trigger are of little concern. > > > > Remove the force_scan stuff, as well as the ugly multi-pass target > > calculation that it necessitated. > > I _really_ like this, I hated the multi-pass part. One thig that I am > worried about and changelog doesn't mention it is what we are going to > do about small (<16MB) memcgs. On one hand they were already ignored in > the global reclaim so this is nothing really new but maybe we want to > preserve the behavior for the memcg reclaim at least which would reduce > side effect of this patch which is a great cleanup otherwise. Or at > least be explicit about this in the changelog.
<16MB groups are a legitimate concern during global reclaim, but we have done it this way for a long time and it never seemed to have mattered in practice.
And for limit reclaim, this should be much less of a concern. It just means we no longer scan these groups at DEF_PRIORITY and will have to increase the scan window. I don't see a problem with that. And that consequence of higher priorities is right in the patch subject.
> Btw. why cannot we simply force scan at least SWAP_CLUSTER_MAX > unconditionally? > > > + /* > > + * If the cgroup's already been deleted, make sure to > > + * scrape out the remaining cache. > Also make sure that small memcgs will not get > unnoticed during the memcg reclaim > > > + */ > > + if (!scan && !mem_cgroup_online(memcg)) > > if (!scan && (!mem_cgroup_online(memcg) || !global_reclaim(sc)))
With this I'd be worried about regressing the setups pointed out in 6f04f48dc9c0 ("mm: only force scan in reclaim when none of the LRUs are big enough.").
Granted, that patch is a little dubious. IMO, we should be steering the LRU balance through references and, in that case in particular, with swappiness. Using the default 60 for zswap is too low.
Plus, I would expect the refault detection code that was introduced around the same time as this patch to counter-act the hot file thrashing that is mentioned in that patch's changelog.
Nevertheless, it seems a bit gratuitous to go against that change so directly when global reclaim hasn't historically been a problem with groups <16MB. Limit reclaim should be fine too.
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