From: Thomas Fjellstrom <thomas@fjellstrom.ca>
To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: kvm@vger.kernel.org
Subject: Re: virtio disk host cpu overhead?
Date: Sat, 11 Mar 2017 09:33:09 -0700 [thread overview]
Message-ID: <2460727.SVSaYC5RoY@natasha> (raw)
In-Reply-To: <20170310015207.GB3213@stefanha-x1.localdomain>
On Friday, March 10, 2017 9:52:07 AM MST Stefan Hajnoczi wrote:
> On Fri, Mar 03, 2017 at 10:05:06AM -0700, Thomas Fjellstrom wrote:
> > I've been testing a new samsung 960 evo nvme drive i just got, and running
> > tests on it in a guest shows a minimum of double the cpu time on the host
> > as it does on the guest. Somehow a guest running a single thread iozone
> > or fio test can use 4+ cores on the host side at 50-100%.
>
> Are you using -drive aio=native?
Yes.
> Are you pinning threads on the host?
No pinning by default.
> If the answer to both questions is no, then QEMU uses a thread pool for
> disk I/O and those threads could be scheduled on any host CPU.
But should they be pegging 4 or more cores, when the host can do the same work
with a single core? I don't mind it spreading work out. But using 4x the cpu
time to do the same work is a bit much.
> Stefan
--
Thomas Fjellstrom
thomas@fjellstrom.ca
next prev parent reply other threads:[~2017-03-11 16:33 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-03 17:05 virtio disk host cpu overhead? Thomas Fjellstrom
2017-03-10 1:52 ` Stefan Hajnoczi
2017-03-11 16:33 ` Thomas Fjellstrom [this message]
2017-03-11 16:45 ` Paolo Bonzini
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