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

[Andrew Morton]

Paul Mackerras <paulus@xxxxxxxxx> wrote:
>
> 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
> applications?

When the workload is a gcc run, the pagefault handler dominates the system
time.  That's the page zeroing.

> 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.

x86's movnta instructions provide a way of initialising memory without
trashing the caches and it has pretty good bandwidth, I believe.  We should
wire that up to these patches and see if it speeds things up.

> 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.

40GB/s.  Is that straight into L1 or does the measurement include writeback?

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