Re: [PATCH] xfs: fix the wrong new_size/rnew_size at xfs_iext_realloc_direct()

[Dave Chinner <david@fromorbit.com>]

On Tue, Sep 24, 2013 at 08:57:30PM +0800, Jeff Liu wrote:
> On 09/24/2013 07:56 AM, Dave Chinner wrote:
> 
> > On Mon, Sep 23, 2013 at 12:47:23PM +0800, Jeff Liu wrote:
> >> Hi Dave,
> >>
> >> On 09/23/2013 08:56 AM, Dave Chinner wrote:
> >>
> >>> On Sun, Sep 22, 2013 at 04:25:15PM +0800, Jeff Liu wrote:
> >>>> From: Jie Liu <jeff.liu@oracle.com>
> >>>>
> >>>> At xfs_iext_realloc_direct(), the new_size is changed by adding
> >>>> if_bytes if originally the extent records are stored at the inline
> >>>> extent buffer, and we have to switch from it to a direct extent
> >>>> list for those new allocated extents, this is wrong. e.g,
....
> >> Actually, what I want to say is that we don't need to perform
> >> "new_size += ifp->if_bytes;" again at xfs_iext_realloc_direct()
> >> because the new_size at xfs_iext_add() already be the size of
> >> extents after adding, just as the variable comments is mentioned.
> > 
> > Yes, I understand.
> > 
> > What I'm really asking is that whether there is any specific impact
> > you can measure as a result of changing the initial allocation size?
> > i.e. are there workloads where there is a measurable difference in
> > memory footprint or noticable performance impact of having to
> > reallocate the direct array more frequently as files grow and/or
> > shrink?
> 
> Not yet observed any performance matter, but IMO this problem can cause
> difference in dynamic memory footprint for creating a large number of
> files with 3 extents and with additional kmalloc/kfree overhead for 4
> extents file.
> 
> For the first case, the current code will allocate buffers from
> kmalloc-128 slab cache rather than kmalloc-64, hence it would occupy
> more memory until being dropped from the cache, e.g,
> 
> # Create 10240 files with 3 extents
> for ((i=0; i<10240; i++))
> do
> 	xfs_io -f -c 'truncate 10m' /xfs/test_$i
> 	xfs_io -c 'pwrite 0 1' /xfs/test_$i 2>&1 >>/dev/null
> 	xfs_io -c 'pwrite 1m 1' /xfs/test_$i 2>&1 >>/dev/null
> 	xfs_io -c 'pwrite 5m 1' /xfs/test_$i 2>&1 >>/dev/null
> done
> 
> # cat /proc/slab_info
> # name        <active_objs>  <num_objs> <objsize> <objperslab> <pagesperslab>...
> 
> # Non-patched -- before creating files
> kmalloc-128         5391   	6176       128		32    		1
> kmalloc-64         21852  	25152      64 		64		1
> 
> # After that -- the number of objects in 128 slab increased significantly, while
> there basically no change in 64 slab
> kmalloc-128        15381  	15488      128		32    		1
> kmalloc-64         21958	25088      64		64    		1
> 
> 
> # patched -- before creating files
> kmalloc-128         5751   	7072	   128		32   		1
> kmalloc-64         21420	24896	   64		64		1	
> 
> After after
> kmalloc-128         6155	6688	   128		32		1
> kmalloc-64         30464	30464	   64		64		1
> 
> With this patch, we can reduce the memory footprint for this particular scenario.

Ok, so it's used the kmalloc-64 slab much more effectively and not
touched the kmalloc-128 slab. Ok, so thats a  measurable difference ;)
> 
> For the 2nd case, i.e, 4 extents file.  It need to resize the direct extent list
> to add the fourth extent because rnew_bytes is be re-initialized to 64 at the
> beginning of xfs_iext_realloc_direct(), however the ifp->if_real_bytes is 128...
...
> # The number of kmalloc calls
> Default     	Patched
> 110364		103471

And that demonstrates the impact in that the array is downsized as
the array grows. Ok, I'm convinced there is a net win here :)

Reviewed-by: Dave Chinner <dchinner@redhat.com>

-- 
Dave Chinner
david@fromorbit.com

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