Re: MMC performance degradation due to too many requests

From: Faiz Abbas
Date: Tue May 15 2018 - 13:40:19 EST

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@xxxxxx> 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.



> - You were previously enabling CONFIG_MMC_BLOCK_BOUNCE