From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from d01relay04.pok.ibm.com (d01relay04.pok.ibm.com [9.56.227.236]) by e5.ny.us.ibm.com (8.12.11.20060308/8.12.11) with ESMTP id k4AJjSjk022481 for ; Wed, 10 May 2006 15:45:28 -0400 Received: from d01av01.pok.ibm.com (d01av01.pok.ibm.com [9.56.224.215]) by d01relay04.pok.ibm.com (8.12.10/NCO/VER6.8) with ESMTP id k4AJjODO238686 for ; Wed, 10 May 2006 15:45:28 -0400 Received: from d01av01.pok.ibm.com (loopback [127.0.0.1]) by d01av01.pok.ibm.com (8.12.11/8.13.3) with ESMTP id k4AJjOQ6005451 for ; Wed, 10 May 2006 15:45:24 -0400 Message-ID: <446242CB.4090106@us.ibm.com> Date: Wed, 10 May 2006 14:45:15 -0500 From: Brian Twichell MIME-Version: 1.0 Subject: Re: [PATCH 0/2][RFC] New version of shared page tables References: <1146671004.24422.20.camel@wildcat.int.mccr.org> <57DF992082E5BD7D36C9D441@[10.1.1.4]> <445FA0CA.4010008@us.ibm.com> <44600F9B.1060207@yahoo.com.au> In-Reply-To: <44600F9B.1060207@yahoo.com.au> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Nick Piggin Cc: Hugh Dickins , Dave McCracken , Linux Memory Management , Linux Kernel List-ID: Nick Piggin wrote: > Brian Twichell wrote: > >> >> If we had to choose between pagetable sharing for small pages and >> hugepages, we would be in favor of retaining pagetable sharing for >> small pages. That is where the discernable benefit is for customers >> that run with "out-of-the-box" settings. Also, there is still some >> benefit there on x86-64 for customers that use hugepages for the >> bufferpools. > > > Of course if it was free performance then we'd want it. The downsides > are that it > is a significant complexity for a pretty small (3%) performance gain > for your apparent > target workload, which is pretty uncommon among all Linux users. Our performance data demonstrated that the potential gain for the non-hugepage case is much higher than 3%. > > Ignoring the complexity, it is still not free. Sharing data across > processes adds to > synchronisation overhead and hurts scalability. Some of these page > fault scalability > scenarios have shown to be important enough that we have introduced > complexity _there_. True, but this needs to be balanced against the fact that pagetable sharing will reduce the number of page faults when it is achieved. Let's say you have N processes which touch all the pages in an M page shared memory region. Without shared pagetables this requires N*M page faults; if pagetable sharing is achieved, only M pagefaults are required. > > And it seems customers running "out-of-the-box" settings really want > to start using > hugepages if they're interested in getting the most performance > possible, no? My perspective is that, once the customer is required to invoke "echo XXX > /proc/sys/vm/nr_hugepages" they've left the "out-of-the-box" domain, and entered the domain of hoping that the number of hugepages is sufficient, because if it's not, they'll probably need to reboot, which can be pretty inconvenient for a production transaction-processing application. Cheers, Brian -- 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