virtio: extend virtio queue size to 256
The goal of the patch is to work around a performance bug in guest
Old linux kernels has a performance flaw in virtio block device access:
on some frequent disk access patterns, e.g. 1M read, the kernel produces
more block requests than needed. This happens because of virtio seg_max
parameter set to 126 (virtqueue_size - 2) which limits the maximum block
request to 516096 (126 * 4096_PAGE_SIZE) bytes.
Setting seg_max > 126 fixes the issue, however, not all linux kernels
allow that without increasing virtio virtqueue size. The old kernels have
a restriction: virtqueue_size >= seg_max. In case of the restriction
violation the old kernels crash.
The restriction is relaxed in the recent linux kernels (ver >= 4.13) with:
Author: Richard W.M. Jones
Date: Thu Aug 10 17:56:51 2017 +0100
scsi: virtio: Reduce BUG if total_sg > virtqueue size to WARN.
and the recent linux kernels don't crash if total_sg > virtqueue size
allowing to set seg_max to the needed value without virtqueue size
To fix the performance flaw in the old linux kernels, it's needed to
increse seg_max to 254, and comply the restriction by setting
virtqueue_size to 256.
This is achievable if seabios can support virtqueue size > 128
which this patch actually does.
Windows kernels don't have virtqueue_size >= seg_max restriction and
isn't affected with this kind of the performance bug.
Signed-off-by: Denis Plotnikov <email@example.com>
Signed-off-by: Gerd Hoffmann <firstname.lastname@example.org>
1 file changed