linux-kernel - RE: [ata] 0568e61225: stress-ng.copy-file.ops_per

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Date:   Tue, 9 Aug 2022 15:16:26 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Damien Le Moal' <damien.lemoal@...nsource.wdc.com>,
        John Garry <john.garry@...wei.com>,
        kernel test robot <oliver.sang@...el.com>
CC:     Christoph Hellwig <hch@....de>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Linux Memory Management List" <linux-mm@...ck.org>,
        "linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
        "lkp@...ts.01.org" <lkp@...ts.01.org>,
        "lkp@...el.com" <lkp@...el.com>,
        "ying.huang@...el.com" <ying.huang@...el.com>,
        "feng.tang@...el.com" <feng.tang@...el.com>,
        "zhengjun.xing@...ux.intel.com" <zhengjun.xing@...ux.intel.com>,
        "fengwei.yin@...el.com" <fengwei.yin@...el.com>
Subject: RE: [ata] 0568e61225: stress-ng.copy-file.ops_per_sec -15.0%
 regression

...
> >> Without knowing what the device adapter is, hard to say where the problem is. I
> >> suspect that with the patch applied, we may be ending up with a small default
> >> max_sectors value, causing overhead due to more commands than necessary.
> >>
> >> Will check what I see with my test rig.
> >
> > As far as I can see, this patch should not make a difference unless the
> > ATA shost driver is setting the max_sectors value unnecessarily low.
> 
> That is my hunch too, hence my question about which host driver is being used
> for this test... That is not apparent from the problem report.

No one's fallen over the old problem and managed to limit
the number of sectors in a read to the number of sectors
in (IIRC) a 'multi sector' read that uses a single DMA burst?

Was always a good way of killing disk performance.

IIRC the maximum number of sectors for an ATA disk transfer is 255.

	David

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