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Date: Wed, 08 Sep 2010 22:29:42 -0500
From: Eric Sandeen <sandeen@...hat.com>
To: Daniel Taylor <Daniel.Taylor@....com>
CC: linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: fix 50% disk write performance regression
Daniel Taylor wrote:
> Just wondering if this patch is adequate or there's more to come.
>
> I want to put a fix into our 2.6.32 kernel.
>
> Thanks.
>
>
I'm working on a couple more things, but I think these 2 fixes are
pretty straightforward if you really want to throw something in
ahead of upstream (I think this applies, hand-edited from my
full series a bit)
I've got a couple other things that seem to keep the writeback
index marching forward properly...
Index: build/fs/ext4/inode.c
===================================================================
--- build.orig/fs/ext4/inode.c
+++ build/fs/ext4/inode.c
@@ -1230,8 +1233,10 @@ static pgoff_t ext4_num_dirty_pages(stru
break;
idx++;
num++;
- if (num >= max_pages)
+ if (num >= max_pages) {
+ done = 1;
break;
+ }
}
pagevec_release(&pvec);
}
@@ -2912,9 +2909,13 @@ static int ext4_da_writepages(struct add
* sbi->max_writeback_mb_bump whichever is smaller.
*/
max_pages = sbi->s_max_writeback_mb_bump << (20 - PAGE_CACHE_SHIFT);
- if (!range_cyclic && range_whole)
- desired_nr_to_write = wbc->nr_to_write * 8;
- else
+
+ if (!range_cyclic && range_whole) {
+ if (wbc->nr_to_write == LLONG_MAX)
+ desired_nr_to_write = wbc->nr_to_write;
+ else
+ desired_nr_to_write = wbc->nr_to_write * 8;
+ } else
desired_nr_to_write = ext4_num_dirty_pages(inode, index,
max_pages);
if (desired_nr_to_write > max_pages)
-Eric
>> -----Original Message-----
>> From: linux-ext4-owner@...r.kernel.org
>> [mailto:linux-ext4-owner@...r.kernel.org] On Behalf Of Eric Sandeen
>> Sent: Monday, August 30, 2010 10:06 PM
>> To: Bill Fink
>> Cc: tytso@....edu; adilger@....com;
>> linux-ext4@...r.kernel.org; bill.fink@...a.gov
>> Subject: Re: [PATCH] ext4: fix 50% disk write performance regression
>>
>> 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.
>>
>> 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
>>
>>
>>
>>
>>> -Thanks a bunch
>>>
>>> -Bill
>>>
>>>
>>>
>>>
>>>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>>>> index 4b8debe..33c2167 100644
>>>> --- a/fs/ext4/inode.c
>>>> +++ b/fs/ext4/inode.c
>>>> @@ -1207,8 +1207,10 @@ static pgoff_t
>>>>
>> ext4_num_dirty_pages(struct inode *inode, pgoff_t idx,
>>
>>>> break;
>>>> idx++;
>>>> num++;
>>>> - if (num >= max_pages)
>>>> - break;
>>>> + if (num >= max_pages) {
>>>> + pagevec_release(&pvec);
>>>> + return num;
>>>> + }
>>>> }
>>>> pagevec_release(&pvec);
>>>> }
>>>> @@ -3002,7 +3004,7 @@ static int ext4_da_writepages(struct
>>>>
>> address_space *mapping,
>>
>>>> * sbi->max_writeback_mb_bump whichever is smaller.
>>>> */
>>>> max_pages = sbi->s_max_writeback_mb_bump << (20 -
>>>>
>> PAGE_CACHE_SHIFT);
>> :
>>
>> --
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>>
>>
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