From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx197.postini.com [74.125.245.197]) by kanga.kvack.org (Postfix) with SMTP id 1ACF96B006C for ; Sat, 29 Sep 2012 10:37:47 -0400 (EDT) Date: Sat, 29 Sep 2012 16:37:37 +0200 From: Andrea Arcangeli Subject: Re: [PATCH 0/3] Virtual huge zero page Message-ID: <20120929143737.GF26989@redhat.com> References: <1348875441-19561-1-git-send-email-kirill.shutemov@linux.intel.com> <20120929134811.GC26989@redhat.com> <20120929143006.GC4110@tassilo.jf.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120929143006.GC4110@tassilo.jf.intel.com> Sender: owner-linux-mm@kvack.org List-ID: To: Andi Kleen Cc: "Kirill A. Shutemov" , Andrew Morton , linux-mm@kvack.org, "H. Peter Anvin" , linux-kernel@vger.kernel.org, "Kirill A. Shutemov" , Arnd Bergmann , Ingo Molnar , linux-arch@vger.kernel.org On Sat, Sep 29, 2012 at 07:30:06AM -0700, Andi Kleen wrote: > On Sat, Sep 29, 2012 at 03:48:11PM +0200, Andrea Arcangeli wrote: > > On Sat, Sep 29, 2012 at 02:37:18AM +0300, Kirill A. Shutemov wrote: > > > Cons: > > > - increases TLB pressure; > > > > I generally don't like using 4k tlb entries ever. This only has the > > From theory I would also prefer the 2MB huge page. > > But some numbers comparing between the two alternatives are definitely > interesting. Numbers are often better than theory. Sure good idea, just all standard benchmarks likely aren't using zero pages so I suggest a basic micro benchmark: some loop of() { memcmp(uninitalized_pointer, (char *)uninitialized_pointer+4G, 4G) barrier(); } > > > There would be a small cache benefit here... but even then some first > > level caches are virtually indexed IIRC (always physically tagged to > > Modern x86 doesn't have virtually indexed caches. With the above memcmp, I'm quite sure the previous patch will beat the new one by a wide margin, especially on modern x86 with more 2M TLB entries and >= 8MB L2 caches. But I agree we need to verify it before taking a decision, and that the numbers are better than theory, or to rephrase it "let's check the theory is right" :) -- 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: email@kvack.org