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

[Denis Plotnikov <dplotnikov@virtuozzo.com>]

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 <den@openvz.org>
>> Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
>> ---
>>   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?

Denis
>
>>       virtio_stw_p(vdev, &blkcfg.geometry.cylinders, conf->cyls);
>>       virtio_stl_p(vdev, &blkcfg.blk_size, blk_size);
>>       virtio_stw_p(vdev, &blkcfg.min_io_size, conf->min_io_size / blk_size);
>> @@ -1272,7 +1272,7 @@ static Property virtio_blk_properties[] = {
>>       DEFINE_PROP_BIT("request-merging", VirtIOBlock, conf.request_merging, 0,
>>                       true),
>>       DEFINE_PROP_UINT16("num-queues", VirtIOBlock, conf.num_queues, 1),
>> -    DEFINE_PROP_UINT16("queue-size", VirtIOBlock, conf.queue_size, 128),
>> +    DEFINE_PROP_UINT16("queue-size", VirtIOBlock, conf.queue_size, 256),
>>       DEFINE_PROP_BOOL("seg-max-adjust", VirtIOBlock, conf.seg_max_adjust, true),
>>       DEFINE_PROP_LINK("iothread", VirtIOBlock, conf.iothread, TYPE_IOTHREAD,
>>                        IOThread *),
>> diff --git a/hw/core/machine.c b/hw/core/machine.c
>> index 2501b540ec..3427d6cf4c 100644
>> --- a/hw/core/machine.c
>> +++ b/hw/core/machine.c
>> @@ -28,6 +28,8 @@
>>   #include "hw/mem/nvdimm.h"
>>   
>>   GlobalProperty hw_compat_4_2[] = {
>> +    { "virtio-blk-device", "queue-size", "128"},
>> +    { "virtio-scsi-device", "virtqueue_size", "128"},
>>       { "virtio-blk-device", "x-enable-wce-if-config-wce", "off" },
>>       { "virtio-blk-device", "seg-max-adjust", "off"},
>>       { "virtio-scsi-device", "seg_max_adjust", "off"},
>> diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
>> index 3b61563609..b38f50a429 100644
>> --- a/hw/scsi/virtio-scsi.c
>> +++ b/hw/scsi/virtio-scsi.c
>> @@ -660,7 +660,7 @@ static void virtio_scsi_get_config(VirtIODevice *vdev,
>>   
>>       virtio_stl_p(vdev, &scsiconf->num_queues, s->conf.num_queues);
>>       virtio_stl_p(vdev, &scsiconf->seg_max,
>> -                 s->conf.seg_max_adjust ? s->conf.virtqueue_size - 2 : 128 - 2);
>> +                 s->conf.seg_max_adjust ? s->conf.virtqueue_size - 2 : 256 - 2);
>>       virtio_stl_p(vdev, &scsiconf->max_sectors, s->conf.max_sectors);
>>       virtio_stl_p(vdev, &scsiconf->cmd_per_lun, s->conf.cmd_per_lun);
>>       virtio_stl_p(vdev, &scsiconf->event_info_size, sizeof(VirtIOSCSIEvent));
>> @@ -965,7 +965,7 @@ static void virtio_scsi_device_unrealize(DeviceState *dev, Error **errp)
>>   static Property virtio_scsi_properties[] = {
>>       DEFINE_PROP_UINT32("num_queues", VirtIOSCSI, parent_obj.conf.num_queues, 1),
>>       DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSI,
>> -                                         parent_obj.conf.virtqueue_size, 128),
>> +                                         parent_obj.conf.virtqueue_size, 256),
>>       DEFINE_PROP_BOOL("seg_max_adjust", VirtIOSCSI,
>>                         parent_obj.conf.seg_max_adjust, true),
>>       DEFINE_PROP_UINT32("max_sectors", VirtIOSCSI, parent_obj.conf.max_sectors,
>> -- 
>> 2.17.0
>>
>>