linux-kernel - Re: MMC performance degradation due to too many requests

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Date:   Tue, 15 May 2018 20:41:02 +0530
From:   Faiz Abbas <faiz_abbas@...com>
To:     Linus Walleij <linus.walleij@...aro.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-omap <linux-omap@...r.kernel.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: MMC performance degradation due to too many requests

Hi Linus,

On Tuesday 15 May 2018 06:09 PM, Linus Walleij wrote:
> On Mon, May 14, 2018 at 1:30 PM, Faiz Abbas <faiz_abbas@...com> wrote:
> 
>> Also, I see many times the number of interrupts for mmc1 in 4.14.
> (...)
>> I also printed out the size of the DMA transfers in the omap_hsmmc
>> driver during the dd command. I see that the maximum size of the dma
>> transfer has decreased from 1 MB to 256kB between 4.13 and 4.14.
> 
> This seems like my fault for removing the bounce buffer.
> (The name bounce buffer is misleading, I suspect the right
> term for what is was doing was "merge buffer" or something,
> coalescing physically consecutive requests.)
> 
> commit de3ee99b097dd51938276e3af388cd4ad0f2750a
> "mmc: Delete bounce buffer handling"
> should be the culprit.

I don't think so. I checked out to the commit before that and see that
the interrupts are still too high.

> 
> The bounce buffer only makes any performance impact if the
> host DMA controller is unable to handle scatter-gather lists
> so the number of segments is set down to 1.
> 
> Can you verifty that:
> - mmc->max_segs == 1

No. It is 64 in v4.14. It was 1024 in v4.13.

v4.14
https://pastebin.ubuntu.com/p/4wXB2jb6zw/

v4.13
https://pastebin.ubuntu.com/p/dpd6kb4YJY/


> - You were previously enabling CONFIG_MMC_BLOCK_BOUNCE

Yes.

Regards,
Faiz