Re: [RESEND] [PATCH] readahead:add blk_run_backing_dev

Vladislav Bolkhovitin
2009-06-30T10:54:54Z

Wu Fengguang, on 06/30/2009 05:04 AM wrote:
> On Mon, Jun 29, 2009 at 11:37:41PM +0800, Vladislav Bolkhovitin wrote:
>> Wu Fengguang, on 06/29/2009 07:01 PM wrote:
>>> On Mon, Jun 29, 2009 at 10:21:24PM +0800, Wu Fengguang wrote:
>>>> On Mon, Jun 29, 2009 at 10:00:20PM +0800, Ronald Moesbergen wrote:
>>>>> ... tests ...
>>>>>
>>>>>> We started with 2.6.29, so why not complete with it (to save additional
>>>>>> Ronald's effort to move on 2.6.30)?
>>>>>>
>>>>>>>> 2. Default vanilla 2.6.29 kernel, 512 KB read-ahead, the rest is default
>>>>>>> How about 2MB RAID readahead size? That transforms into about 512KB
>>>>>>> per-disk readahead size.
>>>>>> OK. Ronald, can you 4 more test cases, please:
>>>>>>
>>>>>> 7. Default vanilla 2.6.29 kernel, 2MB read-ahead, the rest is default
>>>>>>
>>>>>> 8. Default vanilla 2.6.29 kernel, 2MB read-ahead, 64 KB
>>>>>> max_sectors_kb, the rest is default
>>>>>>
>>>>>> 9. Patched by the Fengguang's patch vanilla 2.6.29 kernel, 2MB
>>>>>> read-ahead, the rest is default
>>>>>>
>>>>>> 10. Patched by the Fengguang's patch vanilla 2.6.29 kernel, 2MB
>>>>>> read-ahead, 64 KB max_sectors_kb, the rest is default
>>>>> The results:
>>>> I made a blindless average:
>>>>
>>>> N MB/s IOPS case
>>>>
>>>> 0 114.859 984.148 Unpatched, 128KB readahead, 512 max_sectors_kb
>>>> 1 122.960 981.213 Unpatched, 512KB readahead, 512 max_sectors_kb
>>>> 2 120.709 985.111 Unpatched, 2MB readahead, 512 max_sectors_kb
>>>> 3 158.732 1004.714 Unpatched, 512KB readahead, 64 max_sectors_kb
>>>> 4 159.237 979.659 Unpatched, 2MB readahead, 64 max_sectors_kb
>>>>
>>>> 5 114.583 982.998 Patched, 128KB readahead, 512 max_sectors_kb
>>>> 6 124.902 987.523 Patched, 512KB readahead, 512 max_sectors_kb
>>>> 7 127.373 984.848 Patched, 2MB readahead, 512 max_sectors_kb
>>>> 8 161.218 986.698 Patched, 512KB readahead, 64 max_sectors_kb
>>>> 9 163.908 574.651 Patched, 2MB readahead, 64 max_sectors_kb
>>>>
>>>> So before/after patch:
>>>>
>>>> avg throughput 135.299 => 138.397 by +2.3%
>>>> avg IOPS 986.969 => 903.344 by -8.5%
>>>>
>>>> The IOPS is a bit weird.
>>>>
>>>> Summaries:
>>>> - this patch improves RAID throughput by +2.3% on average
>>>> - after this patch, 2MB readahead performs slightly better
>>>> (by 1-2%) than 512KB readahead
>>> and the most important one:
>>> - 64 max_sectors_kb performs much better than 256 max_sectors_kb, by ~30% !
>> Yes, I've just wanted to point it out ;)
>
> OK, now I tend to agree on decreasing max_sectors_kb and increasing
> read_ahead_kb. But before actually trying to push that idea I'd like
> to
> - do more benchmarks
> - figure out why context readahead didn't help SCST performance
> (previous traces show that context readahead is submitting perfect
> large io requests, so I wonder if it's some io scheduler bug)

Because, as we found out, without your
http://lkml.org/lkml/2009/5/21/319 patch read-ahead was nearly disabled,
hence there were no difference which algorithm was used?

Ronald, can you run the following tests, please? This time with 2 hosts,
initiator (client) and target (server) connected using 1 Gbps iSCSI. It
would be the best if on the client vanilla 2.6.29 will be ran, but any
other kernel will be fine as well, only specify which. Blockdev-perftest
should be ran as before in buffered mode, i.e. with "-a" switch.

1. All defaults on the client, on the server vanilla 2.6.29 with
Fengguang's http://lkml.org/lkml/2009/5/21/319 patch with all default
settings.

2. All defaults on the client, on the server vanilla 2.6.29 with
Fengguang's http://lkml.org/lkml/2009/5/21/319 patch with default RA
size and 64KB max_sectors_kb.

3. All defaults on the client, on the server vanilla 2.6.29 with
Fengguang's http://lkml.org/lkml/2009/5/21/319 patch with 2MB RA size
and default max_sectors_kb.

4. All defaults on the client, on the server vanilla 2.6.29 with
Fengguang's http://lkml.org/lkml/2009/5/21/319 patch with 2MB RA size
and 64KB max_sectors_kb.

5. All defaults on the client, on the server vanilla 2.6.29 with
Fengguang's http://lkml.org/lkml/2009/5/21/319 patch and with context RA
patch. RA size and max_sectors_kb are default. For your convenience I
committed the backported context RA patches into the SCST SVN repository.

6. All defaults on the client, on the server vanilla 2.6.29 with
Fengguang's http://lkml.org/lkml/2009/5/21/319 and context RA patches
with default RA size and 64KB max_sectors_kb.

7. All defaults on the client, on the server vanilla 2.6.29 with
Fengguang's http://lkml.org/lkml/2009/5/21/319 and context RA patches
with 2MB RA size and default max_sectors_kb.

8. All defaults on the client, on the server vanilla 2.6.29 with
Fengguang's http://lkml.org/lkml/2009/5/21/319 and context RA patches
with 2MB RA size and 64KB max_sectors_kb.

9. On the client default RA size and 64KB max_sectors_kb. On the server
vanilla 2.6.29 with Fengguang's http://lkml.org/lkml/2009/5/21/319 and
context RA patches with 2MB RA size and 64KB max_sectors_kb.

10. On the client 2MB RA size and default max_sectors_kb. On the server
vanilla 2.6.29 with Fengguang's http://lkml.org/lkml/2009/5/21/319 and
context RA patches with 2MB RA size and 64KB max_sectors_kb.

11. On the client 2MB RA size and 64KB max_sectors_kb. On the server
vanilla 2.6.29 with Fengguang's http://lkml.org/lkml/2009/5/21/319 and
context RA patches with 2MB RA size and 64KB max_sectors_kb.

(I guess, the results will be interesting not only to us, so I restored
linux-kernel@)

Thanks,
Vlad