Re: [RFC][PATCH] mm: stop balance_dirty_pages doing too much work

Richard Kennedy
2009-08-07T14:36:09Z

On Fri, 2009-08-07 at 14:20 +0200, Peter Zijlstra wrote:
> On Wed, 2009-06-24 at 11:38 +0100, Richard Kennedy wrote:
...
> OK, so Chris ran into this bit yesterday, complaining that he'd only get
> very few write requests and couldn't saturate his IO channel.
>
> Now, since writing out everything once there's something to do sucks for
> Richard, but only writing out stuff when we're over the limit sucks for
> Chris (since we can only be over the limit a little), the best thing
> would be to only write out when we're over the background limit. Since
> that is the low watermark we use for throttling it makes sense that we
> try to write out when above that.
>
> However, since there's a lack of bdi_background_thresh, and I don't
> think introducing one just for this is really justified. How about the
> below?
>
> Chris how did this work for you? Richard, does this make things suck for
> you again?
>
> ---
> mm/page-writeback.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 81627eb..92f42d6 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -545,7 +545,7 @@ static void balance_dirty_pages(struct address_space *mapping)
> * threshold otherwise wait until the disk writes catch
> * up.
> */
> - if (bdi_nr_reclaimable > bdi_thresh) {
> + if (bdi_nr_reclaimable > bdi_thresh/2) {
> writeback_inodes(&wbc);
> pages_written += write_chunk - wbc.nr_to_write;
> get_dirty_limits(&background_thresh, &dirty_thresh,
>
>
I'll run some tests and let you know :)

But what if someone has changed the vm settings?
Maybe something like
(bdi_thresh * dirty_background_ratio / dirty_ratio)
might be better ?

Chris, what sort of workload are you having problems with?

regards
Richard