Re: [PATCH] sbitmap: Use single per-bitmap counting to wake up queued tags

Gabriel Krisman Bertazi
2022-11-09T22:48:23Z

Jens Axboe <axboe@kernel.dk> writes:

> On 11/5/22 5:10 PM, Gabriel Krisman Bertazi wrote:
>> Performance-wise, one should expect very similar performance to the
>> original algorithm for the case where there is no queueing. In both the
>> old algorithm and this implementation, the first thing is to check
>> ws_active, which bails out if there is no queueing to be managed. In the
>> new code, we took care to avoid accounting completions and wakeups when
>> there is no queueing, to not pay the cost of atomic operations
>> unnecessarily, since it doesn't skew the numbers.
>>
>> For more interesting cases, where there is queueing, we need to take
>> into account the cross-communication of the atomic operations. I've
>> been benchmarking by running parallel fio jobs against a single hctx
>> nullb in different hardware queue depth scenarios, and verifying both
>> IOPS and queueing.
>>
>> Each experiment was repeated 5 times on a 20-CPU box, with 20 parallel
>> jobs. fio was issuing fixed-size randwrites with qd=64 against nullb,
>> varying only the hardware queue length per test.
>>
>> queue size 2 4 8 16 32 64
>> 6.1-rc2 1681.1K (1.6K) 2633.0K (12.7K) 6940.8K (16.3K) 8172.3K (617.5K) 8391.7K (367.1K) 8606.1K (351.2K)
>> patched 1721.8K (15.1K) 3016.7K (3.8K) 7543.0K (89.4K) 8132.5K (303.4K) 8324.2K (230.6K) 8401.8K (284.7K)
>>
>> The following is a similar experiment, ran against a nullb with a single
>> bitmap shared by 20 hctx spread across 2 NUMA nodes. This has 40
>> parallel fio jobs operating on the same device
>>
>> queue size 2 4 8 16 32 64
>> 6.1-rc2 1081.0K (2.3K) 957.2K (1.5K) 1699.1K (5.7K) 6178.2K (124.6K) 12227.9K (37.7K) 13286.6K (92.9K)
>> patched 1081.8K (2.8K) 1316.5K (5.4K) 2364.4K (1.8K) 6151.4K (20.0K) 11893.6K (17.5K) 12385.6K (18.4K)
>
> What's the queue depth of these devices? That's the interesting question
> here, as it'll tell us if any of these are actually hitting the slower
> path where you made changes.
>

Hi Jens,

The hardware queue depth is a parameter being varied in this experiment.
Each column of the tables has a different queue depth. Its value is the
first line (queue size) of both tables. For instance, looking at the
first table, for a device with hardware queue depth=2, 6.1-rc2 gave
1681K IOPS and the patched version gave 1721.8K IOPS.

As mentioned, I monitored the size of the sbitmap wqs during the
benchmark execution to confirm it was indeed hitting the slow path and
queueing. Indeed, I observed less queueing on higher QDs (16,32) and
even less for QD=64. For QD<=8, there was extensive queueing present
throughout the execution.

I should provide the queue size over time alongside the latency numbers.
I have to rerun the benchmarks already to collect the information
Chaitanya requested.

> I suspect you are for the second set of numbers, but not for the first
> one?

No. both tables show some level of queueing. The shared bitmap in
table 2 surely has way more intensive queueing, though.

> Anything that isn't hitting the wait path for tags isn't a very useful
> test, as I would not expect any changes there.

Even when there is less to no queueing (QD=64 in this data), we still
enter sbitmap_queue_wake_up and bail out on the first line
!wait_active. This is why I think it is important to include QD=64
here. it is less interesting data, as I mentioned, but it shows no
regressions of the faspath.

Thanks,

--
Gabriel Krisman Bertazi