[Virtio-fs] [QUESTION] A performance problem for buffer write compared with 9p

[wangyan]

On 2019/8/26 20:39, Miklos Szeredi wrote:
> On Sat, Aug 24, 2019 at 10:44 AM wangyan <wangyan122 at huawei.com> wrote:
>
>> According to the result, for "-size=1G", it maybe exceed the dirty pages'
>> upper limit, and it frequently triggered pdflush for write-back. And for
>> "-size=700M", it maybe didn't exceed the dirty pages' upper limit, so no
>> extra pdflush was triggered.
>>
>> But for 9p using "-size=1G", the latency 3.94 usec, and the bandwidth is
>> 2305.5MB/s. It is better than virtiofs using "-size=1G". It seems that
>> it is not affected by the dirty pages' upper limit.
>
> I tried to reproduce these results, but failed to get decent
> (>100MB/s) performance out of 9p.  I don't have fscache set up, does
> that play a part in getting high performance cached writes?
Yes, you should open fscache. My mount command is:
mount -t 9p -o 
trans=virtio,version=9p2000.L,rw,dirsync,nodev,msize=1000000000,cache=fscache 
sharedir /mnt/virtiofs/

Thanks,
Yan Wang
>
> What you describe makes sense, and I have a new patch (attached), but
> didn't see drastic improvement in performance of virtio-fs in my
> tests.
>
> Thanks,
> Miklos
>