RE: [PATCH] x86: introduce memcpy_flushcache_clflushopt

[David Laight <David.Laight@ACULAB.COM>]

From: Mikulas Patocka
> Sent: 18 April 2020 16:21
> 
> On Sat, 18 Apr 2020, David Laight wrote:
> 
> > From: Mikulas Patocka
> > > Sent: 17 April 2020 13:47
> > ...
> > > Index: linux-2.6/drivers/md/dm-writecache.c
> > > ===================================================================
> > > --- linux-2.6.orig/drivers/md/dm-writecache.c	2020-04-17 14:06:35.139999000 +0200
> > > +++ linux-2.6/drivers/md/dm-writecache.c	2020-04-17 14:06:35.129999000 +0200
> > > @@ -1166,7 +1166,10 @@ static void bio_copy_block(struct dm_wri
> > >  			}
> > >  		} else {
> > >  			flush_dcache_page(bio_page(bio));
> > > -			memcpy_flushcache(data, buf, size);
> > > +			if (likely(size > 512))
> > > +				memcpy_flushcache_clflushopt(data, buf, size);
> > > +			else
> > > +				memcpy_flushcache(data, buf, size);
> >
> > Hmmm... have you looked at how long clflush actually takes?
> > It isn't too bad if you just do a small number, but using it
> > to flush large buffers can be very slow.
> 
> Yes, I have. It's here:
> http://people.redhat.com/~mpatocka/testcases/pmem/microbenchmarks/pmem.txt
> 
> sequential write 8 + clflush	- 0.3 GB/s on nvdimm
> sequential write 8 + clflushopt - 1.6 GB/s on nvdimm
> sequential write-nt 8 bytes	- 1.3 GB/s on nvdimm

That table doesn't give enough information to be useful.
The cpu speed, memory speed and transfer lengths are all relevant.

> > I've an Ivy bridge system where the X-server process requests the
> > frame buffer be flushed out every 10 seconds (no idea why).
> > With my 2560x1440 monitor this takes over 3ms.
> >
> > This really needs a cond_resched() every few clflush instructions.
> >
> > 	David
> 
> AFAIK Ivy Bridge doesn't have clflushopt, it only has clflush. clflush
> only allows one outstanding cacle line flush, so it's very slow.
> clflushopt and clwb relaxed this restriction and there can be multiple
> cache-invalidation requests in flight until the user serializes it with
> the sfence instruction.

It isn't that simple.
While clflush on Ivybridge is slower than clflushopt on newer processors
both instructions are (relatively) fast for something like 16 or 32
iterations. After that they get much slower.
I can't remember where I found the relevant figures, even the ones I
found didn't show how large the transfers needed to be before the bytes/sec
became constant.

> The patch checks for clflushopt with
> "static_cpu_has(X86_FEATURE_CLFLUSHOPT)" and if it is not present, it
> falls back to non-temporal stores.

Ok, I was expecting you'd be falling back to clflush first.

	David

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