Re: [Qemu-devel] [PATCH] CPU consumption optimization of 'qemu-img convert' using bdrv_is_allocated()

[Dmitry Konishchev <konishchev@gmail.com>]

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.  Otherwise it is possible to paper over a problem while
> leaving the root cause unsolved.  Avoiding lseek(2) is very important,
> not just for qemu-img but also for running VMs.  lseek(2) should only
> 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=/dev/vda of=/dev/null iflag=direct
>
> I see pread(2) from the posix-aio-compat.c worker threads but no
> lseek(2).  The 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.

-- 
Дмитрий Конищев (Dmitry Konishchev)
mailto:konishchev@gmail.com