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| author | Denis Plotnikov <dplotnikov@virtuozzo.com> | 2019-10-17 17:01:00 +0300 |
|---|---|---|
| committer | Gerd Hoffmann <kraxel@redhat.com> | 2019-10-18 12:07:05 +0200 |
| commit | 50093b4b229b47860c967038e2dc094263e6c9db (patch) | |
| tree | b28dd3da2539eb4b47daf465092d6f44584ce719 | |
| parent | 43f5df79dad6738d52ea79d072de2b56eb96a91f (diff) | |
| download | seabios-50093b4b229b47860c967038e2dc094263e6c9db.tar.gz | |
virtio: extend virtio queue size to 256
The goal of the patch is to work around a performance bug in guest
linux kernels.
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:
commit 44ed8089e991a60d614abe0ee4b9057a28b364e4
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
increasing.
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 <dplotnikov@virtuozzo.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
| -rw-r--r-- | src/hw/virtio-ring.h | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/src/hw/virtio-ring.h b/src/hw/virtio-ring.h index 8604a011..dccc50d7 100644 --- a/src/hw/virtio-ring.h +++ b/src/hw/virtio-ring.h @@ -20,7 +20,7 @@ #define VIRTIO_F_VERSION_1 32 #define VIRTIO_F_IOMMU_PLATFORM 33 -#define MAX_QUEUE_NUM (128) +#define MAX_QUEUE_NUM (256) #define VRING_DESC_F_NEXT 1 #define VRING_DESC_F_WRITE 2 |