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SubjectRe: [PATCH 01/45] writeback: reduce calls to global_page_state in balance_dirty_pages()
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On Tue, 2009-10-13 at 20:12 +0200, Jan Kara wrote:
> > for (;;) {
> > nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
> > global_page_state(NR_UNSTABLE_NFS);
> > nr_writeback = global_page_state(NR_WRITEBACK) +
> > global_page_state(NR_WRITEBACK_TEMP);
> >
> > global_dirty_thresh(&background_thresh, &dirty_thresh);
> >
> > /*
> > * Throttle it only when the background writeback cannot
> > * catch-up. This avoids (excessively) small writeouts
> > * when the bdi limits are ramping up.
> > */
> > if (nr_reclaimable + nr_writeback <
> > (background_thresh + dirty_thresh) / 2)
> > break;
> >
> > bdi_thresh = bdi_dirty_thresh(bdi, dirty_thresh);
> >
> > /*
> > * In order to avoid the stacked BDI deadlock we need
> > * to ensure we accurately count the 'dirty' pages when
> > * the threshold is low.
> > *
> > * Otherwise it would be possible to get thresh+n pages
> > * reported dirty, even though there are thresh-m pages
> > * actually dirty; with m+n sitting in the percpu
> > * deltas.
> > */
> > if (bdi_thresh < 2*bdi_stat_error(bdi)) {
> > bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
> > bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
> > } else {
> > bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
> > bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
> > }
> >
> > /*
> > * The bdi thresh is somehow "soft" limit derived from the
> > * global "hard" limit. The former helps to prevent heavy IO
> > * bdi or process from holding back light ones; The latter is
> > * the last resort safeguard.
> > */
> > dirty_exceeded =
> > (bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh)
> > || (nr_reclaimable + nr_writeback >= dirty_thresh);
> >
> > if (!dirty_exceeded)
> > break;
> >
> > bdi->dirty_exceed_time = jiffies;
> >
> > bdi_writeback_wait(bdi, write_chunk);
> Hmm, probably you've discussed this in some other email but why do we
> cycle in this loop until we get below dirty limit? We used to leave the
> loop after writing write_chunk... So the time we spend in
> balance_dirty_pages() is no longer limited, right?

Wu was saying that without the loop nr_writeback wasn't limited, but
since bdi_writeback_wakeup() is driven from writeout completion, I'm not
sure how again that was so.

We can move all of bdi_dirty to bdi_writeout, if the bdi writeout queue
permits, but it cannot grow beyond the total limit, since we're actually
waiting for writeout completion.

Possibly unstable is peculiar.





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