Re: ext3 sequential read performance (~20%) degrade

[Suparna Bhattacharya <suparna@in.ibm.com>]

On Thu, Sep 14, 2006 at 05:03:08PM -0700, Andrew Morton wrote:
> On Thu, 14 Sep 2006 16:36:12 -0700
> Badari Pulavarty <pbadari@us.ibm.com> wrote:
> 
> > Hi Andrew,
> > 
> > I have been working on tracking down ~20% performance degrade for
> > sequential read performance for ext3. 
> 
> oop.  I'd kinda prefer that we discover things like this before the patch
> gets into mainline.
> 
> > Finally narrowed it down to get_blocks() support. If I force
> > ext3_get_blocks_handle() to always return 1 block - I get better
> > IO rate. I did all the usual stuff, tracked down requests, traced
> > blocksizes, looked at readahead code, looked at mpage_readpages()
> > etc.. I still can't figure out how to explain the degrade..
> > 
> > Any suggestions on how to track it down.
> 
> Learn to driver Jens's blktrace stuff, find out why the IO scheduling went
> bad.
> 
> Number one suspicion: the buffer_boundary() stuff isn't working.

I think you are right about that  - perhaps something along
the lines of the following patch (untested) would help ?

If this is the problem then I guess the degradation should show up for DIO
as well. 

-----------------------------

The boundary block check in ext3_get_blocks_handle needs to be adjusted
against the count of blocks mapped in this call, now that it can map
more than one block.



 linux-2.6.18-rc5-suparna/fs/ext3/inode.c |    2 +-
 1 files changed, 1 insertion(+), 1 deletion(-)

diff -puN fs/ext3/inode.c~ext3-multiblock-boundary-fix fs/ext3/inode.c
--- linux-2.6.18-rc5/fs/ext3/inode.c~ext3-multiblock-boundary-fix	2006-09-15 10:53:12.000000000 +0530
+++ linux-2.6.18-rc5-suparna/fs/ext3/inode.c	2006-09-15 10:54:30.000000000 +0530
@@ -925,7 +925,7 @@ int ext3_get_blocks_handle(handle_t *han
 	set_buffer_new(bh_result);
 got_it:
 	map_bh(bh_result, inode->i_sb, le32_to_cpu(chain[depth-1].key));
-	if (blocks_to_boundary == 0)
+	if (count > blocks_to_boundary)
 		set_buffer_boundary(bh_result);
 	err = count;
 	/* Clean up and exit */

_

Regards
Suparna

-- 
Suparna Bhattacharya (suparna@in.ibm.com)
Linux Technology Center
IBM Software Lab, India