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Re: [PATCH v1 2/4] virtio: increase virtuqueue size for virtio-scsi and

From: Denis Plotnikov
Subject: Re: [PATCH v1 2/4] virtio: increase virtuqueue size for virtio-scsi and virtio-blk
Date: Mon, 10 Feb 2020 18:34:15 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 09.02.2020 10:49, Michael S. Tsirkin wrote:
On Fri, Feb 07, 2020 at 11:48:05AM +0300, Denis Plotnikov wrote:

On 05.02.2020 14:19, Stefan Hajnoczi wrote:
On Tue, Feb 04, 2020 at 12:59:04PM +0300, Denis Plotnikov wrote:
On 30.01.2020 17:58, Stefan Hajnoczi wrote:
On Wed, Jan 29, 2020 at 05:07:00PM +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 guest disk.

More details in the original problem statment:

Suggested-by: Denis V. Lunev <address@hidden>
Signed-off-by: Denis Plotnikov <address@hidden>
    hw/core/machine.c          | 3 +++
    include/hw/virtio/virtio.h | 2 +-
    2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/hw/core/machine.c b/hw/core/machine.c
index 3e288bfceb..8bc401d8b7 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -28,6 +28,9 @@
    #include "hw/mem/nvdimm.h"
    GlobalProperty hw_compat_4_2[] = {
+    { "virtio-blk-device", "queue-size", "128"},
+    { "virtio-scsi-device", "virtqueue_size", "128"},
+    { "vhost-blk-device", "virtqueue_size", "128"},
vhost-blk-device?!  Who has this?  It's not in qemu.git so please omit
this line. ;-)
So in this case the line:

{ "vhost-blk-device", "seg_max_adjust", "off"},

introduced by my patch:

commit 1bf8a989a566b2ba41c197004ec2a02562a766a4
Author: Denis Plotnikov <address@hidden>
Date:   Fri Dec 20 17:09:04 2019 +0300

      virtio: make seg_max virtqueue size dependent

is also wrong. It should be:

{ "vhost-scsi-device", "seg_max_adjust", "off"},

Am I right?
It's just called "vhost-scsi":

include/hw/virtio/vhost-scsi.h:#define TYPE_VHOST_SCSI "vhost-scsi"

On the other hand, do you want to do this for the vhost-user-blk,
vhost-user-scsi, and vhost-scsi devices that exist in qemu.git?  Those
devices would benefit from better performance too.
After thinking about that for a while, I think we shouldn't extend queue
sizes for vhost-user-blk, vhost-user-scsi and vhost-scsi.
This is because increasing the queue sizes seems to be just useless for
them: the all thing is about increasing the queue sizes for increasing
seg_max (it limits the max block query size from the guest). For
virtio-blk-device and virtio-scsi-device it makes sense, since they have
seg-max-adjust property which, if true, sets seg_max to virtqueue_size-2.
vhost-scsi also have this property but it seems the property just doesn't
affect anything (remove it?).
Also vhost-user-blk, vhost-user-scsi and vhost-scsi don't do any seg_max
settings. If I understand correctly, their backends are ment to be
responsible for doing that.
The queue size is set by qemu IIRC.

So, what about changing the queue sizes just for virtio-blk-device and

Hmm that would break ability to migrate between userspace and vhost
backends, would it not?
I'm not sure I've understood what you meant.
Just for the record, I was going to change virtqueue-size for virtio-blk-device and virtio-scsi-device since they can adjust seg_max to the specified queue size and I don't want to touch vhost-s and vhost-user-s since they don't have adjustable seg_max for now.



It seems to be so. We also have the test checking those settings:
For now it checks virtio-scsi-pci and virtio-blk-pci.
I'm going to extend it for the virtqueue size checking.
If I change vhost-user-blk, vhost-user-scsi and vhost-scsi it's worth
to check those devices too. But I don't know how to form a command line
for that 3 devices since they should involve some third party components as
backends (kernel modules, DPDK, etc.) and they seems to be not available in
qemu git.
Is there any way to do it with some qit.qemu available stubs or something
If so, could you please point out the proper way to do it?
qemu.git has contrib/vhost-user-blk/ and contrib/vhost-user-scsi/ if
you need to test those vhost-user devices without external dependencies.


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