Messages in this thread | | | Date | Thu, 21 May 2020 13:23:27 +0100 | From | Chris Down <> | Subject | Re: [PATCH] mm, memcg: reclaim more aggressively before high allocator throttling |
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(I'll leave the dirty throttling discussion to Johannes, because I'm not so familiar with that code or its history.)
Michal Hocko writes: >> > The main problem I see with that approach is that the loop could easily >> > lead to reclaim unfairness when a heavy producer which doesn't leave the >> > kernel (e.g. a large read/write call) can keep a different task doing >> > all the reclaim work. The loop is effectivelly unbound when there is a >> > reclaim progress and so the return to the userspace is by no means >> > proportional to the requested memory/charge. >> >> It's not unbound when there is reclaim progress, it stops when we are within >> the memory.high throttling grace period. Right after reclaim, we check if >> penalty_jiffies is less than 10ms, and abort and further reclaim or >> allocator throttling: > >Just imagine that you have parallel producers increasing the high limit >excess while somebody reclaims those. Sure in practice the loop will be >bounded but the reclaimer might perform much more work on behalf of >other tasks.
A cgroup is a unit and breaking it down into "reclaim fairness" for individual tasks like this seems suspect to me. For example, if one task in a cgroup is leaking unreclaimable memory like crazy, everyone in that cgroup is going to be penalised by allocator throttling as a result, even if they aren't "responsible" for that reclaim.
So the options here are as follows when a cgroup is over memory.high and a single reclaim isn't enough:
1. Decline further reclaim. Instead, throttle for up to 2 seconds. 2. Keep on reclaiming. Only throttle if we can't get back under memory.high.
The outcome of your suggestion to decline further reclaim is case #1, which is significantly more practically "unfair" to that task. Throttling is extremely disruptive to tasks and should be a last resort when we've exhausted all other practical options. It shouldn't be something you get just because you didn't try to reclaim hard enough.
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