Re: [Qemu-devel] Performance problem and improvement about block drive on NFS shares with libnfs

[Jaden Liang <jaden1q84@gmail.com>]

2017-04-01 13:37 GMT+08:00 Fam Zheng <famz@redhat.com>:
> On Sat, 04/01 13:23, Jaden Liang wrote:
>> Hello,
>>
>> I ran qemu with drive file via libnfs recently, and found some performance
>> problem and an improvement idea.
>>
>> I started qemu with 6 drives parameter like nfs://127.0.0.1/dir/vm-disk-x.qcow2
>> which linked to a local NFS server, then used iometer in guest machine to test
>> the 4K random read or random write IO performance. I found that while the IO
>> depth go up, the IOPS hit a bottleneck. I looked into the causes, found that the
>> main thread of qemu used 100% CPU. From the perf data, it show the CPU heats are
>> send / recv calls in libnfs. By reading the source code of libnfs and qemu block
>> drive of nfs.c, libnfs only support single work thread, and the network events
>> of nfs interface in qemu are all registered in the epoll of main thread. That is
>> the cause why main thread uses 100% CPU.
>>
>> After the analysis above, there is an improvement idea comes up. I start a
>> thread for every drive while libnfs open drive file, then create an epoll in
>> every drive thread to handle all of the network events. I have finished an demo
>> modification in block/nfs.c, then rerun iometer in the guest machine, the
>> performance increased a lot. Random read IOPS increases almost 100%, random
>> write IOPS increases about 68%.
>>
>> Test model details
>> VM configure: 6 vdisks in 1 VM
>> Test tool and parameter: iometer with 4K random read and randwrite
>> Backend physical drive: 2 SSDs, 6 vdisks are seperated in 2 SSDs
>>
>> Before modified:
>> IO Depth           1        2          4           8       16         32
>> 4K randread  16659  28387   42932   46868   52108   55760
>> 4K randwrite  12212   19456   30447   30574   35788   39015
>>
>> After modified:
>> IO Depth            1         2          4          8        16          32
>> 4K randread   17661   33115   57138   82016   99369   109410
>> 4K randwrite  12669   21492   36017   51532   61475   65577
>>
>> I could put a up to coding standard patch later. Now I want to get some advise
>> about this modification. Is this a reasonable solution to improve performance in
>> NFS shares? Or there is another better way?
>>
>> Any suggestions would be great! Also please feel free to ask question.
>
> Just one comment: in block/file-posix.c (aio=threads), there is a thread pool
> that does something similar, using the code util/thread-pool.c. Maybe it's
> usable for your block/nfs.c change too.
>
> Also a question: have you considered modifying libnfs to create more worker
> threads? That way all applications using libnfs can benefit.
>
> Fam

Modifying libnfs is also a solution. However, when I looked into
libnfs, found that it
is totally single-thread design. It would be a lot work to make it
support multi-thread mode.
That is why I choose to modify qemu block/nfs.c instead. Because there
are already
some similar ways like file-posix.c.

-- 
Best regards,
Jaden Liang