Re: [PATCH] mm, memcg: reclaim more aggressively before high allocator throttling

[Chris Down]

Chris Down writes:
> > I believe I have asked in other email in this thread. Could you explain
> > why enforcint the requested target (memcg_nr_pages_over_high) is
> > insufficient for the problem you are dealing with? Because that would
> > make sense for large targets to me while it would keep relatively
> > reasonable semantic of the throttling - aka proportional to the memory
> > demand rather than the excess.
>
> memcg_nr_pages_over_high is related to the charge size. As such, if 
> you're way over memory.high as a result of transient reclaim failures, 
> but the majority of your charges are small, it's going to hard to make 
> meaningful progress:
>
> 1. Most nr_pages will be MEMCG_CHARGE_BATCH, which is not enough to help;
> 2. Large allocations will only get a single reclaim attempt to succeed.
>
> As such, in many cases we're either doomed to successfully reclaim a 
> paltry amount of pages, or fail to reclaim a lot of pages. Asking 
> try_to_free_pages() to deal with those huge allocations is generally 
> not reasonable, regardless of the specifics of why it doesn't work in 
> this case.

Oh, I somehow elided the "enforcing" part of your proposal. Still, there's no 
guarantee even if large allocations are reclaimed fully that we will end up 
going back below memory.high, because even a single other large allocation 
which fails to reclaim can knock us out of whack again.