[dm-devel] [PATCH 1/1] block: Convert hd_struct in_flight from atomic to percpu

[Brian King]

On 06/30/2017 06:26 PM, Jens Axboe wrote:
> On 06/30/2017 05:23 PM, Ming Lei wrote:
>> Hi Bian,
>>
>> On Sat, Jul 1, 2017 at 2:33 AM, Brian King <brking at linux.vnet.ibm.com> wrote:
>>> On 06/30/2017 09:08 AM, Jens Axboe wrote:
>>>>>>> Compared with the totally percpu approach, this way might help 1:M or
>>>>>>> N:M mapping, but won't help 1:1 map(NVMe), when hctx is mapped to
>>>>>>> each CPU(especially there are huge hw queues on a big system), :-(
>>>>>>
>>>>>> Not disagreeing with that, without having some mechanism to only
>>>>>> loop queues that have pending requests. That would be similar to the
>>>>>> ctx_map for sw to hw queues. But I don't think that would be worthwhile
>>>>>> doing, I like your pnode approach better. However, I'm still not fully
>>>>>> convinced that one per node is enough to get the scalability we need.
>>>>>>
>>>>>> Would be great if Brian could re-test with your updated patch, so we
>>>>>> know how it works for him at least.
>>>>>
>>>>> I'll try running with both approaches today and see how they compare.
>>>>
>>>> Focus on Ming's, a variant of that is the most likely path forward,
>>>> imho. It'd be great to do a quick run on mine as well, just to establish
>>>> how it compares to mainline, though.
>>>
>>> On my initial runs, the one from you Jens, appears to perform a bit better, although
>>> both are a huge improvement from what I was seeing before.
>>>
>>> I ran 4k random reads using fio to nullblk in two configurations on my 20 core
>>> system with 4 NUMA nodes and 4-way SMT, so 80 logical CPUs. I ran both 80 threads
>>> to a single null_blk as well as 80 threads to 80 null_block devices, so one thread
>>
>> Could you share what the '80 null_block devices' is?  It means you
>> create 80 null_blk
>> devices? Or you create one null_blk and make its hw queue number as 80
>> via module
>> parameter of ''submit_queues"?
> 
> That's a valid question, was going to ask that too. But I assumed that Brian
> used submit_queues to set as many queues as he has logical CPUs in the system.
>>
>> I guess we should focus on multi-queue case since it is the normal way of NVMe.
>>
>>> per null_blk. This is what I saw on this machine:
>>>
>>> Using the Per node atomic change from Ming Lei
>>> 1 null_blk, 80 threads
>>> iops=9376.5K
>>>
>>> 80 null_blk, 1 thread
>>> iops=9523.5K
>>>
>>>
>>> Using the alternate patch from Jens using the tags
>>> 1 null_blk, 80 threads
>>> iops=9725.8K
>>>
>>> 80 null_blk, 1 thread
>>> iops=9569.4K
>>
>> If 1 thread means single fio job, looks the number is too too high, that means
>> one random IO can complete in about 0.1us(100ns) on single CPU, not sure if it
>> is possible, :-)
> 
> It means either 1 null_blk device, 80 threads running IO to it. Or 80 null_blk
> devices, each with a thread running IO to it. See above, he details that it's
> 80 threads on 80 devices for that case.

Right. So the two modes I'm running in are:

1. 80 null_blk devices, each with one submit_queue, with one fio job per null_blk device,
   so 80 threads total. 80 logical CPUs
2. 1 null_blk device, with 80 submit_queues, 80 fio jobs, 80 logical CPUs.

In theory, the two should result in similar numbers. 

Here are the commands and fio configurations:

Scenario #1
modprobe null_blk submit_queues=80 nr_devices=1 irqmode=0

FIO config:
[global]
buffered=0
invalidate=1
bs=4k
iodepth=64
numjobs=80
group_reporting=1
rw=randrw
rwmixread=100
rwmixwrite=0
ioengine=libaio
runtime=60
time_based

[job1]
filename=/dev/nullb0


Scenario #2
modprobe null_blk submit_queues=1 nr_devices=80 irqmode=0

FIO config
[global]
buffered=0
invalidate=1
bs=4k
iodepth=64
numjobs=1
group_reporting=1
rw=randrw
rwmixread=100
rwmixwrite=0
ioengine=libaio
runtime=60
time_based

[job1]
filename=/dev/nullb0
[job2]
filename=/dev/nullb1
[job3]
filename=/dev/nullb2
[job4]
filename=/dev/nullb3
[job5]
filename=/dev/nullb4
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filename=/dev/nullb78
[job80]
filename=/dev/nullb79





-Brian

-- 
Brian King
Power Linux I/O
IBM Linux Technology Center