From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rohit Seth Date: Tue, 08 Nov 2005 18:56:45 +0000 Subject: Re: [RFC] 4-level page table directories. Message-Id: <1131476206.2400.33.camel@akash.sc.intel.com> List-Id: References: <20051027041709.GA13193@attica.americas.sgi.com> In-Reply-To: <20051027041709.GA13193@attica.americas.sgi.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-ia64@vger.kernel.org On Tue, 2005-11-08 at 06:43 -0600, Robin Holt wrote: > On Mon, Nov 07, 2005 at 04:22:32PM -0800, Rohit Seth wrote: > > I think using a 64K page size (may be make that default for IA-64 or > > distribute as another kernel the way SuSE has done) is preferred over > > 4-level page tables....particularly for big memory machines. > > For your particular application, that may be the case. For approx half > of our customers, they _REQUIRE_ their application be certified by the > software vendor. The vendors usually try to limit their exposure by > certifiying on the smallest set kernel/modules/libraries possible. > We don't control that. > I agree with you completely about OSV certification part. And SuSE is again a good example here, they have a released kernel with 64K page size. There is no reason why end customer should not be using this bigger page kernel when desired. > > There is always at least couple of percentage points that an application > > can gain with even smaller memory foot print (like few gig!) by using > > 64K page size for normal pages. > > There is also the possibility that the app may be using the pages sparsely > and therefore wasting a larger percentage of time zeroing memory which > is never needed (smaller percent of page fill). > You are right that there is extra setup cost (+ some additional bloat) coming with default 64K page size. But there is additional cost associated with 4-level page tables too(some of it in the critical low-level fault handlers as well). I just think that we should validate the 64K page size more rigorously. So as to have the OSVs gain more confidence. There is such a wide range of system configurations...having a single kernel configuration may not be the optimal solution. Thanks, -rohit