linux-kernel - Re: [PATCH] mm: make allocation counters per-order

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Message-ID: <CAATkVEyuqQhiL1G=UyOqwABbUGJn2XNvnYpiOp-F3Zb659uOdQ@mail.gmail.com>
Date:   Thu, 6 Jul 2017 10:54:24 -0400
From:   Debabrata Banerjee <dbavatar@...il.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Roman Gushchin <guro@...com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Rik van Riel <riel@...hat.com>, kernel-team@...com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: make allocation counters per-order

On Thu, Jul 6, 2017 at 9:19 AM, Mel Gorman <mgorman@...hsingularity.net> wrote:

> The alloc counter updates are themselves a surprisingly heavy cost to
> the allocation path and this makes it worse for a debugging case that is
> relatively rare. I'm extremely reluctant for such a patch to be added
> given that the tracepoints can be used to assemble such a monitor even
> if it means running a userspace daemon to keep track of it. Would such a
> solution be suitable? Failing that if this is a severe issue, would it be
> possible to at least make this a compile-time or static tracepoint option?
> That way, only people that really need it have to take the penalty.
>
> --
> Mel Gorman

We (Akamai) have been struggling with memory fragmentation issues for
years, and especially the inability to track positive or negative
changes to fragmentation between allocator changes and kernels without
simply looking for how many allocations are failing. We've had someone
toying with trying to report the same data via scanning all pages at
report time versus keeping running stats, although we don't have
working code yet. If it did work it would avoid the runtime overhead.
I don't believe tracepoints are a workable solution for us, since we
would have to be collecting the data from boot, as well as continually
processing the data in userspace at high cost. Ultimately the
locations and other properties (merge-ability) of the allocations in
the buddy groups are also important, which would be interesting to add
on-top of Roman's patch.