On Friday, 10/07/2005 at 11:32 CET, John P Taylor <[EMAIL PROTECTED]> wrote:
> just so I'm clear on this (prior to z/VM 5.2) a guest needs to page > into <2G storage > to do explicit disk I/O and that's as true of 31-bit hosts as 64 bit > hosts, that's useful to know. > What about the contribution to paging from execution of instructions? > What I was > hoping was that from CP's point of view a page will not need to move > below the line > if the code path doesn't request disk I/O - so that for "equavalently > sized 64-bit hosts" there > may be some VM paging reduction depending on the type of work load. Prior to z/VM 5.2, any time CP has to do something with the content of a page (e.g. diagnose parameter lists, virtual I/O), then the page must be moved below 2GB because much of CP still operates in 31-bit addressing mode with a V=R mapping. The addressing mode of the guest is irrelevant. As long as the guest doesn't need the services of CP (i.e. stays in SIE) then the guest pages can stay wherever they are. > Another point I'm not clear on - is if XSTOR is utilised as a page > cache, and CP takes an > interrupt to page in from XSTOR (or even from disk), will it bring in a > page above the 2G line > for 64-bit hosts and below the 2G line for 31-bit hosts? Paging operations are fully 64-bit capable. Only when CP has to read or write data to or from the page is it dragged below the "bar". > I'm trying to understand why I'm seeing 30-50% storage utilisation with > some quite high > XSTOR<->MAIN paging and reasonable MAIN/XSTOR <-> disk paging. I though that > equivalently sized 64-bit hosts might drive up main storage utilisation > and reduce overall > paging. If a large virtual machine is using lots of that memory for I/O buffers, then the pages containing those I/O buffers must be dragged below 2GB, creating contention and easily driving up XSTOR swap. This is why Rob's work on limiting and fixing the pages Linux uses for I/O was so important. z/VM 5.2 changes the way CP views memory and the need for CP to keep dragging pages below 2GB is eliminated. Alan Altmark z/VM Development IBM Endicott ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
