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SubjectRe: [PATCH 1/9] ioatdma: Push pending transactions to hardware more frequently
Chris Leech wrote:
> Every 20 descriptors turns out to be to few append commands with
> newer/faster CPUs. Pushing every 4 still cuts down on MMIO writes to an
> acceptable level without letting the DMA engine run out of work.
>
> Signed-off-by: Chris Leech <christopher.leech@intel.com>
> ---
>
> drivers/dma/ioatdma.c | 4 ++--
> 1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/dma/ioatdma.c b/drivers/dma/ioatdma.c
> index 8e87261..0f77a9d 100644
> --- a/drivers/dma/ioatdma.c
> +++ b/drivers/dma/ioatdma.c
> @@ -310,7 +310,7 @@ static dma_cookie_t do_ioat_dma_memcpy(struct ioat_dma_chan *ioat_chan,
> list_splice_init(&new_chain, ioat_chan->used_desc.prev);
>
> ioat_chan->pending += desc_count;
> - if (ioat_chan->pending >= 20) {
> + if (ioat_chan->pending >= 4) {

This sounds like something that will always be wrong -- or in other
words, always be right for only the latest CPUs. Can this be made
dynamic, based on some timing factor?

Jeff



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