linux-kernel - RE: [PATCH V2] Consider inflight IO in io accounting for high latency devices

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Date:   Tue, 17 Oct 2023 04:13:32 +0000
From:   Gulam Mohamed <gulam.mohamed@...cle.com>
To:     Bart Van Assche <bvanassche@....org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH V2] Consider inflight IO in io accounting for high latency
 devices

Hi Bart,

       Thanks for the review. I agree, for low latency devices if they have single hardware queue, this patch will have significant impact. Can you please let me know about what kind of low latency devices will have a single queue (just for my knowledge)?
Also I would be grateful if you have any suggestions to fix this issue?

Regards,
Gulam Mohamed.

-----Original Message-----
From: Bart Van Assche <bvanassche@....org> 
Sent: Tuesday, October 17, 2023 2:46 AM
To: Gulam Mohamed <gulam.mohamed@...cle.com>; axboe@...nel.dk; linux-block@...r.kernel.org; linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] Consider inflight IO in io accounting for high latency devices

On 10/16/23 13:01, Gulam Mohamed wrote:
> [GULAM]: Yes, it will be called for every submitted bio but for the 
> high latency devices it will not have much impact. This is indicated 
> by the latency figures I provided in the review mail, with and without 
> our patch.

Calling tag iteration code from the hot path is wrong.

There are more devices than only hard disks that have a single hardware queue, e.g. certain SSDs. I'm pretty sure that this patch will have a significant negative performance impact for such devices.

Thanks,

Bart.