linux-kernel - Re: [PATCH v2 2/2] task_mmu: Add user-space support for resetting mm->hiwater

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Date:	Thu, 22 Jan 2015 00:22:14 +0000
From:	Primiano Tucci <primiano@...omium.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Petr Cermak <petrcermak@...omium.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v2 2/2] task_mmu: Add user-space support for resetting
 mm->hiwater_rss (peak RSS)

On Wed, Jan 21, 2015 at 10:58 PM, David Rientjes <rientjes@...gle.com> wrote:
> I think the bigger concern would be that this, and any new line such as
> resettable_hiwater_rss, invalidates itself entirely.  Any process that
> checks the hwm will not know of other processes that reset it, so the
> value itself has no significance anymore.
>  It would just be the mark since the last clear at an unknown time.

How is that different from the current logic of clear_refs and the
corresponding PG_Referenced bit?

> Userspace can monitor the rss of a
> process by reading /proc/pid/stat, there's no need for the kernel to do
> something that userspace can do.

I disagree here. The driving motivation of this patch is precisely the
opposite. There are peak events that last for very short time (order:
10-100 ms) and are practically invisible from user-space (even doing
something awkward like polling in a tight loop). Concrete examples
are: GPU memory transfers, image decoding, compression /
decompression.
These kinds of tasks, which use scratch buffers for few ms, can create
significant (yet short lasting) memory pressure which is desirable to
monitor.
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