linux-kernel - Re: [PATCH v2 3/3] scsi/ncr5380: Improve interrupt latency during PIO tranfers

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Date:   Fri, 09 Sep 2016 07:28:54 -0400
From:   "Martin K. Petersen" <martin.petersen@...cle.com>
To:     Finn Thain <fthain@...egraphics.com.au>
Cc:     "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        <linux-scsi@...r.kernel.org>, <linux-m68k@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Michael Schmitz <schmitzmic@...il.com>
Subject: Re: [PATCH v2 3/3] scsi/ncr5380: Improve interrupt latency during PIO tranfers

>>>>> "Finn" == Finn Thain <fthain@...egraphics.com.au> writes:

Finn> Large PIO transfers are broken up into chunks to try to avoid
Finn> disabling local IRQs for long periods. But IRQs are still disabled
Finn> for too long and this causes SCC FIFO overruns during serial port
Finn> transfers.

Finn> This patch reduces the PIO chunk size to reduce interrupt latency
Finn> to something on the order of milliseconds, at the expense of
Finn> additional CPU overhead from extra iterations of the
Finn> NCR5380_main() loop.

Finn> That CPU overhead is a problem for slow machines (e.g. mac_scsi on
Finn> 25 MHz
Finn> 68030) but these machines generally use PDMA not PIO. This patch
Finn>        doesn't
Finn> make the overhead any worse on my Mac LC III (because it only gets
Finn> about 510 accesses per ms).

Finn> This patch decreases disk performance by a fraction of one percent
Finn> for dmx3191d on my 333 MHz PowerPC 750. Other affected hardware
Finn> (such as g_NCR5380 on x86) was not tested but 5380 ISA cards
Finn> generally use PDMA and not PIO.

Geert?

-- 
Martin K. Petersen	Oracle Linux Engineering