Re: [PATCH] xarray: inline xas_descend to improve performance

[Long Li <leo.lilong@huawei.com>]

On Mon, Apr 15, 2024 at 01:10:53PM -0700, Andrew Morton wrote:
> On Mon, 15 Apr 2024 09:21:36 +0800 Long Li <leo.lilong@huawei.com> wrote:
> 
> > The commit 63b1898fffcd ("XArray: Disallow sibling entries of nodes")
> > modified the xas_descend function in such a way that it was no longer
> > being compiled as an inline function, because it increased the size of
> > xas_descend(), and the compiler no longer optimizes it as inline. This
> > had a negative impact on performance, xas_descend is called frequently
> > to traverse downwards in the xarray tree, making it a hot function.
> > 
> > Inlining xas_descend has been shown to significantly improve performance
> > by approximately 4.95% in the iozone write test.
> > 
> >   Machine: Intel(R) Xeon(R) Gold 6240 CPU @ 2.60GHz
> >   #iozone i 0 -i 1 -s 64g -r 16m -f /test/tmptest
> > 
> > Before this patch:
> > 
> >        kB    reclen    write   rewrite     read    reread
> >  67108864     16384  2230080   3637689 6 315197   5496027
> > 
> > After this patch:
> > 
> >        kB    reclen    write   rewrite     read    reread
> >  67108864     16384  2340360   3666175  6272401   5460782
> > 
> > Percentage change:
> >                        4.95%     0.78%   -0.68%    -0.64%
> > 
> > This patch introduces inlining to the xas_descend function. While this
> > change increases the size of lib/xarray.o, the performance gains in
> > critical workloads make this an acceptable trade-off.
> > 
> > Size comparison before and after patch:
> > .text		.data		.bss		file
> > 0x3502		    0		   0		lib/xarray.o.before
> > 0x3602		    0		   0		lib/xarray.o.after
> > 
> > ...
> >
> > --- a/lib/xarray.c
> > +++ b/lib/xarray.c
> > @@ -200,7 +200,7 @@ static void *xas_start(struct xa_state *xas)
> >  	return entry;
> >  }
> >  
> > -static void *xas_descend(struct xa_state *xas, struct xa_node *node)
> > +static inline void *xas_descend(struct xa_state *xas, struct xa_node *node)
> >  {
> >  	unsigned int offset = get_offset(xas->xa_index, node);
> >  	void *entry = xa_entry(xas->xa, node, offset);
> 
> I thought gcc nowadays treats `inline' as avisory and still makes up
> its own mind?
> 
> Perhaps we should use __always_inline here?

Yes, I agree with you, I will send a new version. thanks!