Re: replace "memset(...,0,PAGE_SIZE)" calls with "clear_page()"?

From: Denis Vlasenko
Date: Sat Dec 30 2006 - 17:45:30 EST

On Saturday 30 December 2006 23:08, Robert P. J. Day wrote:
> >
> > clear_page assumes that given address is page aligned, I think. It
> > may fail if you feed it with misaligned region's address.
> i don't see how that can be true, given that most of the definitions
> of the clear_page() macro are simply invocations of memset(). see for
> yourself:
> $ grep -r "#define clear_page" include
> my only point here was that lots of code seems to be calling memset()
> when it would be clearer to invoke clear_page(). but there's still
> something a bit curious happening here. i'll poke around a bit more
> before i ask, though.

There are MMX implementations of clear_page().

I was experimenting with SSE[2] clear_page() which uses
non-temporal stores. That one requires 16 byte alignment.

BTW, it worked ~300% faster than memset. But Andi Kleen
insists that cache eviction caused by NT stores will make it
slower in macrobenchmark.

Apart from fairly extensive set of microbechmarks
I tested kernel compiles (i.e. "real world load")
and they are FASTER too, not slower, but Andi
is fairly entrenched in his opinion ;)
I gave up.
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