From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Thu, 23 Dec 2004 13:37:45 -0800 From: Andrew Morton Subject: Re: Prezeroing V2 [0/3]: Why and When it works Message-Id: <20041223133745.1d95bb08.akpm@osdl.org> In-Reply-To: <16843.13418.630413.64809@cargo.ozlabs.ibm.com> References: <41C20E3E.3070209@yahoo.com.au> <16843.13418.630413.64809@cargo.ozlabs.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Paul Mackerras Cc: clameter@sgi.com, linux-ia64@vger.kernel.org, torvalds@osdl.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: Paul Mackerras 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? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: aart@kvack.org