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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Jann Horn <jannh@google.com>
Cc: kernel test robot <rong.a.chen@intel.com>,
	Christoph Hellwig <hch@lst.de>, Oleg Nesterov <oleg@redhat.com>,
	Kirill Shutemov <kirill@shutemov.name>, Jan Kara <jack@suse.cz>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	lkp@lists.01.org
Subject: Re: [gup] 17839856fd: stress-ng.vm-splice.ops_per_sec 2158.6% improvement
Date: Mon, 15 Jun 2020 12:09:58 -0700	[thread overview]
Message-ID: <CAHk-=wgAH-yXX=1T_hqenVVmQPVKP-dkRKKkHYcWAXK0tRzz6Q@mail.gmail.com> (raw)
In-Reply-To: <CAG48ez1v1b4X5LgFya6nvi33-TWwqna_dc5jGFVosqQhdn_Nkg@mail.gmail.com>

On Mon, Jun 15, 2020 at 12:33 AM Jann Horn <jannh@google.com> wrote:
>
> So in summary I guess the test was just really slow up until now
> because it was hitting a slowpath that you wouldn't hit during normal
> usage? At least for vmsplice(), writing uninitialized pages doesn't
> really make a whole lot of sense...

Heh. My main worry with that commit was that we'd see the reverse
effect: somebody implicitly depending on the continued sharing of
pages in some odd splice situation, and where breaking the COW would
result in a performance degradation.

The fact that it went the other way instead is somewhat ironic.

But good.

I guess I'll just ping the stable people to say "yeah, put that in
stable too". It's a fix, and apparently it _helps_ performance for the
degenerate cases rather than hurting anything.

             Linus

WARNING: multiple messages have this Message-ID (diff)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: lkp@lists.01.org
Subject: Re: [gup] 17839856fd: stress-ng.vm-splice.ops_per_sec 2158.6% improvement
Date: Mon, 15 Jun 2020 12:09:58 -0700	[thread overview]
Message-ID: <CAHk-=wgAH-yXX=1T_hqenVVmQPVKP-dkRKKkHYcWAXK0tRzz6Q@mail.gmail.com> (raw)
In-Reply-To: <CAG48ez1v1b4X5LgFya6nvi33-TWwqna_dc5jGFVosqQhdn_Nkg@mail.gmail.com>

[-- Attachment #1: Type: text/plain, Size: 858 bytes --]

On Mon, Jun 15, 2020 at 12:33 AM Jann Horn <jannh@google.com> wrote:
>
> So in summary I guess the test was just really slow up until now
> because it was hitting a slowpath that you wouldn't hit during normal
> usage? At least for vmsplice(), writing uninitialized pages doesn't
> really make a whole lot of sense...

Heh. My main worry with that commit was that we'd see the reverse
effect: somebody implicitly depending on the continued sharing of
pages in some odd splice situation, and where breaking the COW would
result in a performance degradation.

The fact that it went the other way instead is somewhat ironic.

But good.

I guess I'll just ping the stable people to say "yeah, put that in
stable too". It's a fix, and apparently it _helps_ performance for the
degenerate cases rather than hurting anything.

             Linus

  parent reply	other threads:[~2020-06-15 19:10 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-11  4:04 [gup] 17839856fd: stress-ng.vm-splice.ops_per_sec 2158.6% improvement kernel test robot
2020-06-11  4:04 ` kernel test robot
2020-06-11 20:24 ` Linus Torvalds
2020-06-11 20:24   ` Linus Torvalds
2020-06-12  0:07   ` [LKP] " Philip Li
2020-06-12  0:07     ` Philip Li
2020-06-15  6:58     ` [LKP] " Feng Tang
2020-06-15  6:58       ` Feng Tang
2020-06-15  7:32   ` Jann Horn
2020-06-15  7:32     ` Jann Horn
2020-06-15  8:33     ` [LKP] " Feng Tang
2020-06-15  8:33       ` Feng Tang
2020-06-15 19:09     ` Linus Torvalds [this message]
2020-06-15 19:09       ` Linus Torvalds

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