Re: clear_user_highpage()

["David S. Miller" <davem@redhat.com>]

On Wed, 11 Aug 2004 19:18:18 -0700 (PDT)
Linus Torvalds <torvalds@osdl.org> wrote:

> I really do believe (but can't back it up with any real numbers) that we 
> want to try to keep pages in cache as long as possible. That means keeping 
> the pages close to the last CPU that used them, btw.

So I did some testing.

I changed the cache-bypassing clear_user_page() into one that uses
normal stores and does allocate in the L2 cache.

I ran the full build tests 3 times for each case, and the numbers were
consistent.  It makes the full build take a full minute longer.

And I truly believe this is because of the argument William and myself
are making, that a write protection fault does not mean the process is
going to access a majority of the data in that page any time soon at all.

{clear,copy}_user_page() is not some kind of "prefetch the whole page
into the cache" for the user.  It would be if the user would access
the entire thing in the near future, but I do not believe that is
the typical access pattern for fresh anonymous pages.