linux-kernel - Re: CFQ idling kills I/O performance on ext4 with blkio cgroup controller

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Date:   Wed, 22 May 2019 03:01:24 -0700
From:   "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>
To:     Paolo Valente <paolo.valente@...aro.org>
Cc:     linux-fsdevel@...r.kernel.org,
        linux-block <linux-block@...r.kernel.org>,
        linux-ext4@...r.kernel.org, cgroups@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        jmoyer@...hat.com, Theodore Ts'o <tytso@....edu>,
        amakhalov@...are.com, anishs@...are.com, srivatsab@...are.com
Subject: Re: CFQ idling kills I/O performance on ext4 with blkio cgroup
 controller

On 5/22/19 2:09 AM, Paolo Valente wrote:
> 
> First, thank you very much for testing my patches, and, above all, for
> sharing those huge traces!
> 
> According to the your traces, the residual 20% lower throughput that you
> record is due to the fact that the BFQ injection mechanism takes a few
> hundredths of seconds to stabilize, at the beginning of the workload.
> During that setup time, the throughput is equal to the dreadful ~60-90 KB/s
> that you see without this new patch.  After that time, there
> seems to be no loss according to the trace.
> 
> The problem is that a loss lasting only a few hundredths of seconds is
> however not negligible for a write workload that lasts only 3-4
> seconds.  Could you please try writing a larger file?
> 

I tried running dd for longer (about 100 seconds), but still saw around
1.4 MB/s throughput with BFQ, and between 1.5 MB/s - 1.6 MB/s with
mq-deadline and noop. But I'm not too worried about that difference.

> In addition, I wanted to ask you whether you measured BFQ throughput
> with traces disabled.  This may make a difference.
> 

The above result (1.4 MB/s) was obtained with traces disabled.

> After trying writing a larger file, you can try with low_latency on.
> On my side, it causes results to become a little unstable across
> repetitions (which is expected).
> 
With low_latency on, I get between 60 KB/s - 100 KB/s.

Regards,
Srivatsa
VMware Photon OS