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Subject: Re: [PATCH 0/2] Limit number of hw queues by nr_cpu_ids for
 virtio-blk and virtio-scsi
To:     linux-scsi@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-block@vger.kernel.org
References: <1553682995-5682-1-git-send-email-dongli.zhang@oracle.com>
Cc:     mst@redhat.com, jasowang@redhat.com, axboe@kernel.dk,
        jejb@linux.ibm.com, martin.petersen@oracle.com, cohuck@redhat.com,
        linux-kernel@vger.kernel.org
From:   Dongli Zhang <dongli.zhang@oracle.com>
Message-ID: <8ee49e44-a52c-0596-e18e-7d92da8cef8f@oracle.com>
Date:   Mon, 8 Apr 2019 21:57:46 +0800
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ping?

Thank you very much!

Dongli Zhang

On 03/27/2019 06:36 PM, Dongli Zhang wrote:
> When tag_set->nr_maps is 1, the block layer limits the number of hw queues
> by nr_cpu_ids. No matter how many hw queues are use by
> virtio-blk/virtio-scsi, as they both have (tag_set->nr_maps == 1), they
> can use at most nr_cpu_ids hw queues.
> 
> In addition, specifically for pci scenario, when the 'num-queues' specified
> by qemu is more than maxcpus, virtio-blk/virtio-scsi would not be able to
> allocate more than maxcpus vectors in order to have a vector for each
> queue. As a result, they fall back into MSI-X with one vector for config
> and one shared for queues.
> 
> Considering above reasons, this patch set limits the number of hw queues
> used by nr_cpu_ids for both virtio-blk and virtio-scsi.
> 
> -------------------------------------------------------------
> 
> Here is test result of virtio-scsi:
> 
> qemu cmdline:
> 
> -smp 2,maxcpus=4, \
> -device virtio-scsi-pci,id=scsi0,num_queues=8, \
> -device scsi-hd,drive=drive0,bus=scsi0.0,channel=0,scsi-id=0,lun=0, \
> -drive file=test.img,if=none,id=drive0
> 
> Although maxcpus=4 and num_queues=8, 4 queues are used while 2 interrupts
> are allocated.
> 
> # cat /proc/interrupts
> ... ...
>  24:          0          0   PCI-MSI 65536-edge      virtio0-config
>  25:          0        369   PCI-MSI 65537-edge      virtio0-virtqueues
> ... ...
> 
> # /sys/block/sda/mq/
> 0  1  2  3   ------> 4 queues although qemu sets num_queues=8
> 
> 
> With the patch set, there is per-queue interrupt.
> 
> # cat /proc/interrupts
>  24:          0          0   PCI-MSI 65536-edge      virtio0-config
>  25:          0          0   PCI-MSI 65537-edge      virtio0-control
>  26:          0          0   PCI-MSI 65538-edge      virtio0-event
>  27:        296          0   PCI-MSI 65539-edge      virtio0-request
>  28:          0        139   PCI-MSI 65540-edge      virtio0-request
>  29:          0          0   PCI-MSI 65541-edge      virtio0-request
>  30:          0          0   PCI-MSI 65542-edge      virtio0-request
> 
> # ls /sys/block/sda/mq
> 0  1  2  3
> 
> -------------------------------------------------------------
> 
> Here is test result of virtio-blk:
> 
> qemu cmdline:
> 
> -smp 2,maxcpus=4,
> -device virtio-blk-pci,drive=drive-virtio-disk0,id=virtio-disk0,num-queues=8
> -drive test.img,format=raw,if=none,id=drive-virtio-disk0
> 
> Although maxcpus=4 and num-queues=8, 4 queues are used while 2 interrupts
> are allocated.
> 
> # cat /proc/interrupts
> ... ...
>  24:          0          0   PCI-MSI 65536-edge      virtio0-config
>  25:          0         65   PCI-MSI 65537-edge      virtio0-virtqueues
> ... ...
> 
> # ls /sys/block/vda/mq
> 0  1  2  3    -------> 4 queues although qemu sets num_queues=8
> 
> 
> With the patch set, there is per-queue interrupt.
> 
> # cat /proc/interrupts
>  24:          0          0   PCI-MSI 65536-edge      virtio0-config
>  25:         64          0   PCI-MSI 65537-edge      virtio0-req.0
>  26:          0      10290   PCI-MSI 65538-edge      virtio0-req.1
>  27:          0          0   PCI-MSI 65539-edge      virtio0-req.2
>  28:          0          0   PCI-MSI 65540-edge      virtio0-req.3
> 
> # ls /sys/block/vda/mq/
> 0  1  2  3
> 
> 
> Reference: https://lore.kernel.org/lkml/e4afe4c5-0262-4500-aeec-60f30734b4fc@default/
> 
> Thank you very much!
> 
> Dongli Zhang
>