Re: Prezeroing V2 [0/3]: Why and When it works

From: Paul Mackerras
Date: Thu Dec 23 2004 - 16:12:33 EST

Christoph Lameter writes:

> The most expensive operation in the page fault handler is (apart of SMP
> locking overhead) the zeroing of the page.

Re-reading this I see that you mean the zeroing of the page that is
mapped into the process address space, not the page table pages. So
ignore my previous reply.

Do you have any statistics on how often a page fault needs to supply a
page of zeroes versus supplying a copy of an existing page, for real

In any case, unless you have magic page-zeroing hardware, I am still
inclined to think that zeroing the page at the time of the fault is
the most efficient, since that means the page will be hot in the cache
for the process to use. If you zero it earlier using CPU stores, it
can only cause more overall memory traffic, as far as I can see.

I did some measurements once on my G5 powermac (running a ppc64 linux
kernel) of how long clear_page takes, and it only takes 96ns for a 4kB
page. This is real-life elapsed time in the kernel, not just some
cache-hot benchmark measurement. Thus I don't think your patch will
gain us anything on ppc64.

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