* Re: [PATCH] AHCI: Remove an unnecessary flush from ahci_qc_issue
[not found] <1215350600-5179-1-git-send-email-matthew@wil.cx>
@ 2008-07-06 13:44 ` Jeff Garzik
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From: Jeff Garzik @ 2008-07-06 13:44 UTC (permalink / raw)
To: Matthew Wilcox; +Cc: linux-ide, Matthew Wilcox, LKML
Matthew Wilcox wrote:
> In an I/O heavy workload (IOZone), ahci_qc_issue is the second-highest
> consumer of CPU cycles. Removing the flush gets us approximately 10%
> bandwidth improvement. I believe this to be because the CPU can start
> queueing the next request instead of waiting for the readl() to flush the
> writes to the device. The flush isn't necessary because we're using a
> 'queue' metaphor; we don't guarantee the command has got to the device,
> nor do we need to guarantee the command has got to the controller.
>
> Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
> ---
> drivers/ata/ahci.c | 1 -
> 1 files changed, 0 insertions(+), 1 deletions(-)
>
> diff --git a/drivers/ata/ahci.c b/drivers/ata/ahci.c
> index 6a7a70a..58915bd 100644
> --- a/drivers/ata/ahci.c
> +++ b/drivers/ata/ahci.c
> @@ -1846,7 +1846,6 @@ static unsigned int ahci_qc_issue(struct ata_queued_cmd *qc)
> if (qc->tf.protocol == ATA_PROT_NCQ)
> writel(1 << qc->tag, port_mmio + PORT_SCR_ACT);
> writel(1 << qc->tag, port_mmio + PORT_CMD_ISSUE);
> - readl(port_mmio + PORT_CMD_ISSUE); /* flush */
>
(LKML CC added for wider review)
As I noted in IRC, I've queued this and am planning to apply this, as
I've been thinking along the same lines for quite a while now... not
just in this driver but other drivers too.
A couple places in libata arguably need additional flushing, but some
places could actually stand to use /less/
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