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@xxxxxxxxxxxxxxx> --- 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 */ return 0; } -- 1.5.5.4 -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
