Re: [PATCH] fix readahead breakage for sequential after random reads

From: Ram Pai (linuxram_at_us.ibm.com)
Date: 08/04/04

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To: Andrew Morton <akpm@osdl.org>
Date:	04 Aug 2004 10:38:37 -0700

On Mon, 2004-07-26 at 21:18, Ram Pai wrote:
> On Mon, 2004-07-26 at 17:08, Andrew Morton wrote:
> > Ram Pai <linuxram@us.ibm.com> wrote:
> > >
> > > Andrew,
> > > Yes the patch fixes a valid bug.
> > >
> >
> > Please don't top-post :(
> > > RP
> > >
> > > On Mon, 2004-07-26 at 16:29, Andrew Morton wrote:
> > > > Miklos Szeredi <miklos@szeredi.hu> wrote:
> > > > >
> > > > > Current readahead logic is broken when a random read pattern is
> > > > > followed by a long sequential read. The cause is that on a window
> > > > > miss ra->next_size is set to ra->average, but ra->average is only
> > > > > updated at the end of a sequence, so window size will remain 1 until
> > > > > the end of the sequential read.
> > > > >
> > > > > This patch fixes this by taking the current sequence length into
> > > > > account (code taken from towards end of page_cache_readahead()), and
> > > > > also setting ra->average to a decent value in handle_ra_miss() when
> > > > > sequential access is detected.
> > > >
> > > > Thanks. Do you have any performance testing results from this patch?
> > > >
> > > Ram Pai <linuxram@us.ibm.com> wrote:
> > >
> > > Andrew,
> > > Yes the patch fixes a valid bug.
> >
> > Fine, but the readahead code is performance-sensitive, and it takes quite
> > some time for any regressions to be discovered. So I'm going to need to
> > either sit on this patch for a very long time, or extensively test it
> > myself, or await convincing test results from someone else.
> >
> > Can you help with that?
>
> yes I will run all my standard testsuites before we take this patch.
> (DSS workload, iozone, sysbench). I will get back with some results
> sooon. Probably by the end of this week.

Ok I have enclosed the results. The summary is: there is no significant
improvement or decrease in performance of (DSS workload, iozone,
sysbench) The increase or decrease is in the margin of errors.

I have also enclosed a patch that partially backs off Miklos's fix.
Shane Shrybman correctly pointed out that the real fix is to set
ra->average value to max/2 when we move from readahead-off mode to
readahead-on mode. The other part of Miklos's fix becomes irrelevent.

Sorry it took some time to get back on this. Its almost automated so
turnaround time should be quick now-on-wards.

--- linux-2.6.8-rc3/mm/readahead.c 2004-08-03 14:26:46.000000000 -0700
+++ mypatchdir/mm/readahead.c 2004-08-04 17:08:47.665196424 -0700
@@ -468,11 +468,7 @@ do_io:
                           * pages shall be accessed in the next
                           * current window.
                           */
- average = ra->average;
- if (ra->serial_cnt > average)
- average = (ra->serial_cnt + ra->average + 1) / 2;
-
- ra->next_size = min(average , (unsigned long)max);
+ ra->next_size = min(ra->average , (unsigned long)max);
                 }
                 ra->start = offset;
                 ra->size = ra->next_size;

DSS WORKLOAD
----------------

---------------------------------------------------------------------
| 2.6.8-rc2 | miklos_patch | miklos_mod_patch |
| | | |
----------------------------------------------------------------------
| | | |
| X | 0.9001% increase| 0.73649% increase |
| | w.r.t X | w.r.t X |
| | | |
---------------------------------------------------------------------

                        iozone NFS
                        -------
                iozone -c -t1 -s 4096m -r 128k -w

---------------------------------------------------------------------------
| | 2.6.8-rc2 | miklos_patch | miklos_mod_patch|
|---------------|---------------|-----------------------|-----------------|
| | | | |
|Sequential | 2272.77KB/sec | 2266.45KB/sec | 2269.98KB/sec |
| | | | |
|reverse read | 3078.14KB/sec | 3066.73KB/sec | 3079.87KB/sec |
| | | | |
|stride read | 3253.49KB/sec | 3220.70KB/sec | 3264.41KB/sec |
| | | | |
|random read | 5225.44KB/sec | 5231.68KB/sec | 5274.32KB/sec |
| | | | |
|random mix | 8389.64KB/sec | 8458.40KB/sec | 7722.72KB/sec |
| | | | |
|pread | 2264.64KB/sec | 2264.35KB/sec | 2256.15KB/sec |
--------------------------------------------------------------------------

sysbench
--------

sysbench --num-threads=256 --test=fileio --file-total-size=2800M --file-test-mode=rndrw run

-------------------------------------------------------------------------
| | 2.6.8-rc2 | miklos_patch |miklos_mod_patch|
-------------------------------------------------------------------------
| | | | |
| Throughput | 2.745MB/sec | 2.721MB/sec | 2.607MB/sec |
|---------------|-----------------------|---------------|----------------|
| | | | |
| | | | |
|Requests/sec | 175.99 | 174.40MB/sec | 167.08MB/sec |
|---------------|-----------------------|---------------|----------------|
| | | | |
| | | | |
|Execution time | 56.8213s | 57.3400s | 59.8498s |
--------------------------------------------------------------------------

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