linux-kernel - Re: [PATCH] iosched: Add i10 I/O Scheduler

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Date:   Fri, 13 Nov 2020 14:03:05 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Sagi Grimberg <sagi@...mberg.me>,
        Rachit Agarwal <rach4x0r@...il.com>,
        Christoph Hellwig <hch@....de>
Cc:     linux-block@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Keith Busch <kbusch@...nel.org>,
        Ming Lei <ming.lei@...hat.com>,
        Jaehyun Hwang <jaehyun.hwang@...nell.edu>,
        Qizhe Cai <qc228@...nell.edu>,
        Midhul Vuppalapati <mvv25@...nell.edu>,
        Rachit Agarwal <ragarwal@...cornell.edu>,
        Sagi Grimberg <sagi@...htbitslabs.com>,
        Rachit Agarwal <ragarwal@...nell.edu>
Subject: Re: [PATCH] iosched: Add i10 I/O Scheduler

On 11/13/20 1:34 PM, Sagi Grimberg wrote:
> 
>> I haven't taken a close look at the code yet so far, but one quick note
>> that patches like this should be against the branches for 5.11. In fact,
>> this one doesn't even compile against current -git, as
>> blk_mq_bio_list_merge is now called blk_bio_list_merge.
> 
> Ugh, I guess that Jaehyun had this patch bottled up and didn't rebase
> before submitting.. Sorry about that.
> 
>> In any case, I did run this through some quick peak testing as I was
>> curious, and I'm seeing about 20% drop in peak IOPS over none running
>> this. Perf diff:
>>
>>      10.71%     -2.44%  [kernel.vmlinux]  [k] read_tsc
>>       2.33%     -1.99%  [kernel.vmlinux]  [k] _raw_spin_lock
> 
> You ran this with nvme? or null_blk? I guess neither would benefit
> from this because if the underlying device will not benefit from
> batching (at least enough for the extra cost of accounting for it) it
> will be counter productive to use this scheduler.

This is nvme, actual device. The initial posting could be a bit more
explicit on the use case, it says:

"For NVMe SSDs, the i10 I/O scheduler achieves ~60% improvements in
terms of IOPS per core over "noop" I/O scheduler."

which made me very skeptical, as it sounds like it's raw device claims.

Does beg the question of why this is a new scheduler then. It's pretty
basic stuff, something that could trivially just be added a side effect
of the core (and in fact we have much of it already). Doesn't really seem
to warrant a new scheduler at all. There isn't really much in there.

>>> [5] https://github.com/i10-kernel/upstream-linux/blob/master/dss-evaluation.pdf
>>
>> Was curious and wanted to look it up, but it doesn't exist.
> 
> I think this is the right one:
> https://github.com/i10-kernel/upstream-linux/blob/master/i10-evaluation.pdf
> 
> We had some back and forth around the naming, hence this was probably
> omitted.

That works, my local results were a bit worse than listed in there though.
And what does this mean:

"We note that Linux I/O scheduler introduces an additional kernel worker
thread at the I/O dispatching stage"

It most certainly does not for the common/hot case.

-- 
Jens Axboe