On Tue, 31 Aug 2010, Eric Sandeen wrote:
> Bill Fink wrote:
> > On Mon, 30 Aug 2010, Eric Sandeen wrote:
> >
> >> Can you give this a shot?
> >>
> >> The first hunk is, I think, the biggest problem. Even if
> >> we get the max number of pages we need, we keep scanning forward
> >> until "done" without doing any more actual, useful work.
> >>
> >> The 2nd hunk is an oddity, some places assign nr_to_write
> >> to LONG_MAX, and we get here and multiply -that- by 8... giving
> >> us "-8" for nr_to_write, that can't help things when we
> >> do later comparisons on that number...
> >>
> >> I also see us asking to find pages starting at "idx" and
> >> the first dirty page we find is well ahead of that,
> >> I'm not sure if that's indicative of a problem or not.
> >>
> >> Anyway, want to give this a shot, in place of the patch you sent,
> >> and see how it fares compared to stock and/or with your patch?
> >>
> >> It's build-and-sanity tested but not really performance tested here.
> >>
> >> Thanks,
> >> -Eric
> >
> > Great! It looks like that does the trick.
> >
> > 2.6.35 + your patch:
> >
> > i7test7% dd if=/dev/zero of=/i7raid/bill/testfile1 bs=1M count=32768
> > 32768+0 records in
> > 32768+0 records out
> > 34359738368 bytes (34 GB) copied, 50.6702 s, 678 MB/s
> >
> > That's the same performance as with my patch, and pretty darn
> > close to the original 2.6.31 performance.
>
> hah, that's good esp. considering my followup email that found
> what I think is a problem with my patch. ;)
>
> What happens if you change:
>
> if (!range_cyclic && range_whole && wbc->nr_to_write != LONG_MAX)
> desired_nr_to_write = wbc->nr_to_write * 8;
> else
> desired_nr_to_write = ext4_num_dirty_pages(inode, index,
>
> to:
>
> if (!range_cyclic && range_whole) {
> if (wbc->nr_to_write != LONG_MAX)
> desired_nr_to_write = wbc->nr_to_write * 8;
> else
> desired_nr_to_write = wbc->nr_to_write;
> } else
> desired_nr_to_write = ext4_num_dirty_pages(inode, index,
>
> and see how that fares? I think that makes a little more sense, if we
> got there with LONG_MAX that means "write everything" and there's no need
> to bump it up or to go counting pages. It may not make any real difference.
That's also fine.
-Bill
> But I'm seeing really weird behavior in writeback, it starts out nicely
> writing 32768 pages at a time, and then goes all wonky, revisiting pages
> it's already done and doing IO in little chunks. This is going to take
> some staring I think.
>
> -Eric
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