Re: Re: [patch 2/2 v3]raid5: create multiple threads to handle stripes

["Jianpeng Ma" <majianpeng@gmail.com>]

On 2012-08-15 16:04 Shaohua Li <shli@kernel.org> Wrote:
>2012/8/15 Jianpeng Ma <majianpeng@gmail.com>:
>> On 2012-08-15 11:51 Shaohua Li <shli@kernel.org> Wrote:
>>>2012/8/14 Jianpeng Ma <majianpeng@gmail.com>:
>>>> On 2012-08-13 10:20 Shaohua Li <shli@kernel.org> Wrote:
>>>>>2012/8/13 Shaohua Li <shli@kernel.org>:
>>>>>> On Mon, Aug 13, 2012 at 09:06:45AM +0800, Jianpeng Ma wrote:
>>>>>>> On 2012-08-13 08:21 Shaohua Li <shli@kernel.org> Wrote:
>>>>>>> >2012/8/11 Jianpeng Ma <majianpeng@gmail.com>:
>>>>>>> >> On 2012-08-09 16:58 Shaohua Li <shli@kernel.org> Wrote:
>>>>>>> >>>This is a new tempt to make raid5 handle stripes in multiple threads, as
>>>>>>> >>>suggested by Neil to have maxium flexibility and better numa binding. It
>>>>>>> >>>basically is a combination of my first and second generation patches. By
>>>>>>> >>>default, no multiple thread is enabled (all stripes are handled by raid5d).
>>>>>>> >>>
>>>>>>> >>>An example to enable multiple threads:
>>>>>>> >>>#echo 3 > /sys/block/md0/md/auxthread_number
>>>>>>> >>>This will create 3 auxiliary threads to handle stripes. The threads can run
>>>>>>> >>>on any cpus and handle stripes produced by any cpus.
>>>>>>> >>>
>>>>>>> >>>#echo 1-3 > /sys/block/md0/md/auxth0/cpulist
>>>>>>> >>>This will bind auxiliary thread 0 to cpu 1-3, and this thread will only handle
>>>>>>> >>>stripes produced by cpu 1-3. User tool can further change the thread's
>>>>>>> >>>affinity, but the thread can only handle stripes produced by cpu 1-3 till the
>>>>>>> >>>sysfs entry is changed again.
>>>>>>> >>>
>>>>>>> >>>If stripes produced by a CPU aren't handled by any auxiliary thread, such
>>>>>>> >>>stripes will be handled by raid5d. Otherwise, raid5d doesn't handle any
>>>>>>> >>>stripes.
>>>>>>> >>>
>>>>>>> >> I tested and found two problem(maybe not).
>>>>>>> >>
>>>>>>> >> 1:print cpulist of auxth, you maybe lost print the '\n'.
>>>>>>> >> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
>>>>>>> >> index 7c8151a..3700cdc 100644
>>>>>>> >> --- a/drivers/md/raid5.c
>>>>>>> >> +++ b/drivers/md/raid5.c
>>>>>>> >> @@ -4911,9 +4911,13 @@ struct raid5_auxth_sysfs {
>>>>>>> >>  static ssize_t raid5_show_thread_cpulist(struct mddev *mddev,
>>>>>>> >>         struct raid5_auxth *thread, char *page)
>>>>>>> >>  {
>>>>>>> >> +       int n;
>>>>>>> >>         if (!mddev->private)
>>>>>>> >>                 return 0;
>>>>>>> >> -       return cpulist_scnprintf(page, PAGE_SIZE, &thread->work_mask);
>>>>>>> >> +       n = cpulist_scnprintf(page, PAGE_SIZE - 2, &thread->work_mask);
>>>>>>> >> +       page[n++] = '\n';
>>>>>>> >> +       page[n] = 0;
>>>>>>> >> +       return n;
>>>>>>> >>  }
>>>>>>> >>
>>>>>>> >>  static ssize_t
>>>>>>> >
>>>>>>> >some sysfs entries print out '\n', some not, I don't mind add it
>>>>>>> I search kernel code found places which like this print out '\n';
>>>>>>> Can you tell rule which use or not?
>>>>>>> Thanks!
>>>>>>
>>>>>> I'm not aware any rule about this
>>>>>>
>>>>>>> >> 2: Test 'dd if=/dev/zero of=/dev/md0 bs=2M ', the performance regress remarkable.
>>>>>>> >> auxthread_number=0, 200MB/s;
>>>>>>> >> auxthread_number=4, 95MB/s.
>>>>>>> >
>>>>>>> >So multiple threads handle stripes reduce request merge. In your
>>>>>>> >workload, raid5d isn't a bottleneck at all. In practice, I thought only
>>>>>>> >array which can drive high IOPS needs enable multi thread. And
>>>>>>> >if you create multiple threads, better let the threads handle different
>>>>>>> >cpus.
>>>>>>> I will test for multiple threads.
>>>>>> Thanks
>>>> I used fio for randwrite test using four thread which run different cpus.
>>>> The bs is 4k/8k/16k.
>>>> The result isn't increase regardless of whether using authread(four authread which run different cpu) or not?
>>>> Maybe my test config had problem?
>>>
>>>how fast is your raid? If your raid can't drive high IOPS, it's
>>>not strange multithread makes no difference.
>>>
>> Only 175 for 4K. I think your patch for harddisk dose not effect.
>> Maybe it's only for ssd.
>>>>>BTW, can you try below patch for the above dd workload?
>>>>>http://git.kernel.dk/?p=linux-block.git;a=commitdiff;h=274193224cdabd687d804a26e0150bb20f2dd52c
>>>>>That one is reverted in upstream, but eventually we should make it
>>>>>enter again after some CFQ issues are fixed.
>>>> I tested this patch.And not found problem.And the performance did not increase.
>>>
>>>Ok, each thread delivers request in random time, so merge doesn't
>>>work even with that patch. I didn't worry about big size request too
>>>much, since if you set correct affinity for the auxthread, the issue
>>>should go away. And mulithread is for fast storage, I suppose it has
>>>no advantages for harddisk raid. On the other hand, maybe we can
>>>make MAX_STRIPE_BATCH bigger. Currently it's 8, so the auxthread
>>>will dispatch 8*4k request for the workload. Changing it to 16
>>>(16*4=64k) should be good enough even for hard disk raid.
>>>
>> I review your code and have a question about wakeup authread:
>>>static void raid5_wakeup_stripe_thread(struct stripe_head *sh)
>>>{
>>>       struct r5conf *conf = sh->raid_conf;
>>>       struct raid5_percpu *percpu;
>>>       int i, orphaned = 1;
>>>
>>>       percpu = per_cpu_ptr(conf->percpu, sh->cpu);
>>>       for_each_cpu(i, &percpu->handle_threads) {
>>>               md_wakeup_thread(conf->aux_threads[i]->thread);
>>>               orphaned = 0;
>>>       }
>> If there are small stripes in handle_threads of cpu0.But the authread0/1 can run cpu0.
>> It's no necessary to wakup all thread.authread0 may exec all stripe,but the authread1 only wakeup and sleep,but it will spin_lock_irq(&conf->device_lock).
>> I think you should add some limited to do .
>
>I used to have a counter for stripes queued, with it, we can determine
>how many threads should be waken up. That is what I did when each
>each thread can handle stripes from any CPU. I thought this problem
>isn't sever now since each thread can just handle strips from one or
>several CPUs. If this really is a problem, we can fix it later, but my test
>doesn't show this is a problem.
Yes, you provide the interface for user.
Are there debuginfo for user to find info to control?
Because i didn't have ssd device,so the test about this patch can't do anymore.