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Re: [PATCH v2] virtio: increase virtuqueue size for virtio-scsi and virt

From: Denis Plotnikov
Subject: Re: [PATCH v2] virtio: increase virtuqueue size for virtio-scsi and virtio-blk
Date: Thu, 13 Feb 2020 15:41:18 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 13.02.2020 14:45, Stefan Hajnoczi wrote:
On Thu, Feb 13, 2020 at 12:28:25PM +0300, Denis Plotnikov wrote:

On 13.02.2020 12:08, Stefan Hajnoczi wrote:
On Thu, Feb 13, 2020 at 11:08:35AM +0300, Denis Plotnikov wrote:
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:

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?
Argh!  Good point :-).

How about a seg_max_default property that is initialized to 254 for
modern machines and 126 to old machines?
Hmm, but we'll achieve the same but with more code changes, don't we?
254 is because the queue-size is 256. We gonna leave 128-2 for older machine
just for not breaking anything. All other seg_max adjustment is provided by
seg_max_adjust which is "on" by default in modern machine types.

to summarize:

modern mt defaults:
seg_max_adjust = on
queue_size = 256

=> default seg_max = 254
=> changing queue-size will change seg_max = queue_size - 2

old mt defaults:
seg_max_adjust = off
queue_size = 128

=> default seg_max = 126
=> changing queue-size won't change seg_max, it's always = 126 like it was
You're right!  The only strange case is a modern machine type with
seg_max_adjust=off, where queue_size will be 256 but seg_max will be
126.  But no user would want to disable seg_max_adjust, so it's okay.

I agree with you that the line of code can remain unchanged:

    * Only old machine types use seg_max_adjust=off and there the default
    * value of queue_size is 128.
   virtio_stl_p(vdev, &blkcfg.seg_max,
                s->conf.seg_max_adjust ? s->conf.queue_size - 2 : 128 - 2);

Ok, I'll resend the patch sortly


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