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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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