Re: [Qemu-devel] [PATCH v3 3/5] qemu-file: Add fast ftell code path

[Eric Blake <eblake@redhat.com>]

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On 12/26/2014 07:42 AM, Alexander Graf wrote:
> For ftell we flush the output buffer to ensure that we don't have anything
> lingering in our internal buffers. This is a very safe thing to do.
> 
> However, with the dynamic size measurement that the dynamic vmstate
> description will bring this would turn out quite slow.
> 
> Instead, we can fast path this specific measurement and just take the
> internal buffers into account when telling the kernel our position.
> 
> I'm sure I overlooked some corner cases where this doesn't work, so
> instead of tuning the safe, existing version, this patch adds a fast
> variant of ftell that gets used by the dynamic vmstate description code
> which isn't critical when it fails.
> 
> Signed-off-by: Alexander Graf <agraf@suse.de>
> 
> ---
> 
> v2 -> v3:
> 
>   - improve ftell_fast, now works with bdrv too
> ---
>  include/migration/qemu-file.h |  1 +
>  migration/qemu-file.c         | 16 ++++++++++++++++
>  2 files changed, 17 insertions(+)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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