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Date: Wed, 15 Jun 2011 17:14:47 +0400
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From: Dmitry Konishchev <konishchev@gmail.com>
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Subject: Re: [Qemu-devel] [PATCH] CPU consumption optimization of 'qemu-img
 convert' using bdrv_is_allocated()
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org

On Wed, Jun 15, 2011 at 4:02 PM, Stefan Hajnoczi <stefanha@gmail.com> wrote=
:
> We need to fully understand performance before applying optimizations
> on top. =C2=A0Otherwise it is possible to paper over a problem while
> leaving the root cause unsolved. =C2=A0Avoiding lseek(2) is very importan=
t,
> not just for qemu-img but also for running VMs. =C2=A0lseek(2) should onl=
y
> be invoked if the image is growable/removable.
>
> When I run a VM from a virtio-blk raw image I see no lseek(2) calls.
>
> On the host:
> strace -p $pid_of_qemu -f
>
> Inside the guest:
> dd if=3D/dev/vda of=3D/dev/null iflag=3Ddirect
>
> I see pread(2) from the posix-aio-compat.c worker threads but no
> lseek(2). =C2=A0The total_sectors cached value is being used.
>
> Does strace(1) show lseek(2) on your host?

No, I don't see lseek() in the strace output. But in the other hand I
see that bdrv_get_geometry() uses bdrv_getlength() which is
implemented using lseek() in block/raw-posix.c...
May be I'am mistaken about lseek(), but I get 9% slower version if
disable caching.

--=20
=D0=94=D0=BC=D0=B8=D1=82=D1=80=D0=B8=D0=B9 =D0=9A=D0=BE=D0=BD=D0=B8=D1=89=
=D0=B5=D0=B2 (Dmitry Konishchev)
mailto:konishchev@gmail.com