Re: [PATCH] blk-mq: allow hardware queue to get more tag while sharing a tag set

["yukuai (C)" <yukuai3@huawei.com>]

On 2021/08/04 2:38, Bart Van Assche wrote:
> On 8/2/21 7:57 PM, yukuai (C) wrote:
>> The cpu I'm testing is Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz, and
>> after switching to io_uring with "--thread --gtod_reduce=1
>> --ioscheduler=none", the numbers can increase to 330k, yet still
>> far behind 6000k.
> 
> On 
> https://ark.intel.com/content/www/us/en/ark/products/120485/intel-xeon-gold-6140-processor-24-75m-cache-2-30-ghz.html 
> I found the following information about that CPU:
> 18 CPU cores
> 36 hyperthreads
> 
> so 36 fio jobs should be sufficient. Maybe IOPS are lower than expected 
> because of how null_blk has been configured? This is the configuration 
> that I used in my test:
> 
> modprobe null_blk nr_devices=0 &&
>      udevadm settle &&
>      cd /sys/kernel/config/nullb &&
>      mkdir nullb0 &&
>      cd nullb0 &&
>      echo 0 > completion_nsec &&
>      echo 512 > blocksize &&
>      echo 0 > home_node &&
>      echo 0 > irqmode &&
>      echo 1024 > size &&
>      echo 0 > memory_backed &&
>      echo 2 > queue_mode &&
>      echo 1 > power ||
>      exit $?

hi Bart,

After applying this configuration, the number of null_blk in my
machine is about 650k(330k before). Is this still too low?

By the way, there are no performance degradation.

Thanks
Kuai
> 
>> The new atomic operations in the hot path is atomic_read() from
>> hctx_may_queue(), and the atomic variable will change in two
>> situations:
>>
>> a. fail to get driver tag with dbusy not set, increase and set dbusy.
>> b. if dbusy is set when queue switch from busy to dile, decrease and
>> clear dbusy.
>>
>> During the period a device "idle -> busy -> idle", the new atomic
>> variable can be writen twice at most, which means this is almost
>> readonly in the above test situation. So I guess the impact on
>> performance is minimal ?
> 
> Please measure the performance impact of your patch.
> 
> Thanks,
> 
> Bart.
> 
> .
>