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SubjectRe: [PATCH 0/7] Per-bdi writeback flusher threads
[CC Jens]

On Tue, Apr 07, 2009 at 10:03:38PM +0800, Jos Houtman wrote:
>
> I tried the write-back branch from the 2.6-block tree.
>
> And I can atleast confirm that it works, atleast in relation to the
> writeback not keeping up when the device was congested before it wrote a
> 1024 pages.
>
> See: http://lkml.org/lkml/2009/3/22/83 for a bit more information.

Hi Jos, you said that this simple patch solved the problem, however you
mentioned somehow suboptimal performance. Can you elaborate that? So
that I can push or improve it.

Thanks,
Fengguang
---
fs/fs-writeback.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
--- mm.orig/fs/fs-writeback.c
+++ mm/fs/fs-writeback.c
@@ -325,7 +325,8 @@ __sync_single_inode(struct inode *inode,
* soon as the queue becomes uncongested.
*/
inode->i_state |= I_DIRTY_PAGES;
- if (wbc->nr_to_write <= 0) {
+ if (wbc->nr_to_write <= 0 ||
+ wbc->encountered_congestion) {
/*
* slice used up: queue for next turn
*/
> But the second problem seen in that thread, a write-starve-read problem does
> not seem to solved. In this problem the writes of the writeback algorithm
> starve the ongoing reads, no matter what io-scheduler is picked.
>
> For good measure I also applied the blk-latency patches on top of the
> writeback branch, this did not improve anything. Nor did lowering
> max_sectors_kb, as linus suggested in the IO latency thread.
>
>
> As for a reproducible test-case: the simplest I could come up with was
> modifying the fsync-tester not to fsync, but letting the normal writeback
> handle it. And starting a separate process that tries to sequentially read a
> file from the same device. The read performance drops to a bare minimum as
> soon as the writeback algorithm kicks in.
>
>
> Jos
>
>
>
>


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