linux-ext4 - Re: [PATCH] ext4: fix 50% disk write performance regression

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Date:	Tue, 31 Aug 2010 00:53:09 -0400
From:	Bill Fink <billfink@...dspring.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	tytso@....edu, adilger@....com, linux-ext4@...r.kernel.org,
	bill.fink@...a.gov
Subject: Re: [PATCH] ext4: fix 50% disk write performance regression

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.

						-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);
> -	if (!range_cyclic && range_whole)
> +	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,
--
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