On 12.02.2020 18:43, Stefan Hajnoczi wrote:
On Tue, Feb 11, 2020 at 05:14:14PM +0300, Denis Plotnikov wrote:
The goal is to reduce the amount of requests issued by a guest on
1M reads/writes. This rises the performance up to 4% on that kind of
disk access pattern.
The maximum chunk size to be used for the guest disk accessing is
limited with seg_max parameter, which represents the max amount of
pices in the scatter-geather list in one guest disk request.
Since seg_max is virqueue_size dependent, increasing the virtqueue
size increases seg_max, which, in turn, increases the maximum size
of data to be read/write from a guest disk.
More details in the original problem statment:
https://lists.gnu.org/archive/html/qemu-devel/2017-12/msg03721.html
Suggested-by: Denis V. Lunev <address@hidden>
Signed-off-by: Denis Plotnikov <address@hidden>
---
hw/block/virtio-blk.c | 4 ++--
hw/core/machine.c | 2 ++
hw/scsi/virtio-scsi.c | 4 ++--
3 files changed, 6 insertions(+), 4 deletions(-)
diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 09f46ed85f..6df3a7a6df 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -914,7 +914,7 @@ static void virtio_blk_update_config(VirtIODevice *vdev,
uint8_t *config)
memset(&blkcfg, 0, sizeof(blkcfg));
virtio_stq_p(vdev, &blkcfg.capacity, capacity);
virtio_stl_p(vdev, &blkcfg.seg_max,
- s->conf.seg_max_adjust ? s->conf.queue_size - 2 : 128 - 2);
+ s->conf.seg_max_adjust ? s->conf.queue_size - 2 : 256 - 2);
This value must not change on older machine types.
Yes, that's true, but ..
So does this patch
need to turn seg-max-adjust *on* in hw_compat_4_2 so that old machine
types get 126 instead of 254?
If we set seg-max-adjust "on" in older machine types, the setups using them
and having queue_sizes set , for example, 1024 will also set seg_max to 1024
- 2 which isn't the expected behavior: older mt didn't change seg_max in
that case and stuck with 128 - 2.
So, should we, instead, leave the default 128 - 2, for seg_max?