linux-kernel - Re: Linux 4.15-rc6

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Message-ID: <20180103125724.GA2189@1wt.eu>
Date:   Wed, 3 Jan 2018 13:57:25 +0100
From:   Willy Tarreau <w@....eu>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andres Freund <andres@...razel.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 4.15-rc6

On Tue, Jan 02, 2018 at 01:09:13PM -0800, Linus Torvalds wrote:
> On Tue, Jan 2, 2018 at 12:28 PM, Andres Freund <andres@...razel.de> wrote:
> >
> > I thought it'd be interesting to run a short benchmark to be able to
> > estimate the impact of the PTI work on postgres workloads (which I work
> > on). On my skylake laptop, a memory resident, OLTP workload with 16
> > connections results in:
> 
> Yeah, that's actually pretty much in line with expectations.
> 
> Something around 5% performance impact of the isolation is what people
> are looking at.
> 
> Obviously it depends on just exactly what you do. Some loads will
> hardly be affected at all, if they just spend all their time in user
> space. And if you do a lot of small system calls, you might see
> double-digit slowdowns.

I can confirm, I've just run some tests on haproxy on a core i7-4790K
and I'm observing a performance loss of ~17%, making the connection
rate go down from 245k/s to 204k/s. It's indeed quite significant for
such use cases, eventhough I think it might reasonably be absorbed by
usual noise in most use cases.

With that said, I think we should start to think about an option to
disable this per process. We could imagine for example a prctl()
requiring CAP_SYS_ADMIN to disable it. This would at least allow
processes started as root to disable it when they consider themselves
irrelevant to this kind of protection (mostly I/O intensive or network
intensive applications).

> > This isn't a complaint, I just thought it might be useful
> > information. If it helps for anything/anybody, I'm happy to run
> > additional benchmarks / provide additional information.
> 
> Note that it will depend heavily on the hardware too. Older CPU's
> without PCID will be impacted more by the isolation.

Interesting. This CPU has PCID, so it's possible that older hardware
may indeed be hit a bit more.

Regards,
Willy