From: Stefano Stabellini <sstabellini@kernel.org>
To: Juergen Gross <jgross@suse.com>
Cc: xen-devel <xen-devel@lists.xenproject.org>,
Stefano Stabellini <sstabellini@kernel.org>,
Ian Jackson <ian.jackson@eu.citrix.com>,
Wei Liu <wei.liu2@citrix.com>
Subject: Re: [for-4.9] Re: HVM guest performance regression
Date: Fri, 26 May 2017 12:01:56 -0700 (PDT) [thread overview]
Message-ID: <alpine.DEB.2.10.1705261201010.18759@sstabellini-ThinkPad-X260> (raw)
In-Reply-To: <2e6d9143-1abe-2f99-155f-f1071245ca41@suse.com>
On Fri, 26 May 2017, Juergen Gross wrote:
> On 26/05/17 18:19, Ian Jackson wrote:
> > Juergen Gross writes ("HVM guest performance regression"):
> >> Looking for the reason of a performance regression of HVM guests under
> >> Xen 4.7 against 4.5 I found the reason to be commit
> >> c26f92b8fce3c9df17f7ef035b54d97cbe931c7a ("libxl: remove freemem_slack")
> >> in Xen 4.6.
> >>
> >> The problem occurred when dom0 had to be ballooned down when starting
> >> the guest. The performance of some micro benchmarks dropped by about
> >> a factor of 2 with above commit.
> >>
> >> Interesting point is that the performance of the guest will depend on
> >> the amount of free memory being available at guest creation time.
> >> When there was barely enough memory available for starting the guest
> >> the performance will remain low even if memory is being freed later.
> >>
> >> I'd like to suggest we either revert the commit or have some other
> >> mechanism to try to have some reserve free memory when starting a
> >> domain.
> >
> > Oh, dear. The memory accounting swamp again. Clearly we are not
> > going to drain that swamp now, but I don't like regressions.
> >
> > I am not opposed to reverting that commit. I was a bit iffy about it
> > at the time; and according to the removal commit message, it was
> > basically removed because it was a piece of cargo cult for which we
> > had no justification in any of our records.
> >
> > Indeed I think fixing this is a candidate for 4.9.
> >
> > Do you know the mechanism by which the freemem slack helps ? I think
> > that would be a prerequisite for reverting this. That way we can have
> > an understanding of why we are doing things, rather than just
> > flailing at random...
>
> I wish I would understand it.
>
> One candidate would be 2M/1G pages being possible with enough free
> memory, but I haven't proofed this yet. I can have a try by disabling
> big pages in the hypervisor.
Right, if I had to bet, I would put my money on superpages shattering
being the cause of the problem.
> What makes the whole problem even more mysterious is that the
> regression was detected first with SLE12 SP3 (guest and dom0, Xen 4.9
> and Linux 4.4) against older systems (guest and dom0). While trying
> to find out whether the guest or the Xen version are the culprit I
> found that the old guest (based on kernel 3.12) showed the mentioned
> performance drop with above commit. The new guest (based on kernel
> 4.4) shows the same bad performance regardless of the Xen version or
> amount of free memory. I haven't found the Linux kernel commit yet
> being responsible for that performance drop.
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next prev parent reply other threads:[~2017-05-26 19:02 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-26 16:14 HVM guest performance regression Juergen Gross
2017-05-26 16:19 ` [for-4.9] " Ian Jackson
2017-05-26 17:00 ` Juergen Gross
2017-05-26 19:01 ` Stefano Stabellini [this message]
2017-05-29 19:05 ` Juergen Gross
2017-05-30 7:24 ` Jan Beulich
[not found] ` <592D3A3A020000780015D787@suse.com>
2017-05-30 10:33 ` Juergen Gross
2017-05-30 10:43 ` Jan Beulich
[not found] ` <592D68DC020000780015D919@suse.com>
2017-05-30 14:57 ` Juergen Gross
2017-05-30 15:10 ` Jan Beulich
2017-06-06 13:44 ` Juergen Gross
2017-06-06 16:39 ` Stefano Stabellini
2017-06-06 19:00 ` Juergen Gross
2017-06-06 19:08 ` Stefano Stabellini
2017-06-07 6:55 ` Juergen Gross
2017-06-07 18:19 ` Stefano Stabellini
2017-06-08 9:37 ` Juergen Gross
2017-06-08 18:09 ` Stefano Stabellini
2017-06-08 18:28 ` Juergen Gross
2017-06-08 21:00 ` Dario Faggioli
2017-06-11 2:27 ` Konrad Rzeszutek Wilk
2017-06-12 5:48 ` Solved: " Juergen Gross
2017-06-12 7:35 ` Andrew Cooper
2017-06-12 7:47 ` Juergen Gross
2017-06-12 8:30 ` Andrew Cooper
2017-05-26 17:04 ` Dario Faggioli
2017-05-26 17:25 ` Juergen Gross
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