Re: [PATCH 15/15] fuse: Increase MAX_WRITE_SIZE with a second buffer

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Mar 25, 2025 at 05:06:55PM +0100, Hanna Czenczek wrote:
> We probably want to support larger write sizes than just 4k; 64k seems
> nice.  However, we cannot read partial requests from the FUSE FD, we
> always have to read requests in full; so our read buffer must be large
> enough to accommodate potential 64k writes if we want to support that.
> 
> Always allocating FuseRequest objects with 64k buffers in them seems
> wasteful, though.  But we can get around the issue by splitting the
> buffer into two and using readv(): One part will hold all normal (up to
> 4k) write requests and all other requests, and a second part (the
> "spill-over buffer") will be used only for larger write requests.  Each
> FuseQueue has its own spill-over buffer, and only if we find it used
> when reading a request will we move its ownership into the FuseRequest
> object and allocate a new spill-over buffer for the queue.
> 
> This way, we get to support "large" write sizes without having to
> allocate big buffers when they aren't used.
> 
> Also, this even reduces the size of the FuseRequest objects because the
> read buffer has to have at least FUSE_MIN_READ_BUFFER (8192) bytes; but
> the requests we support are not quite so large (except for >4k writes),
> so until now, we basically had to have useless padding in there.
> 
> With the spill-over buffer added, the FUSE_MIN_READ_BUFFER requirement
> is easily met and we can decrease the size of the buffer portion that is
> right inside of FuseRequest.
> 
> As for benchmarks, the benefit of this patch can be shown easily by
> writing a 4G image (with qemu-img convert) to a FUSE export:
> - Before this patch: Takes 25.6 s (14.4 s with -t none)
> - After this patch: Takes 4.5 s (5.5 s with -t none)
> 
> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
> ---
>  block/export/fuse.c | 137 ++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 118 insertions(+), 19 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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