Hi Christoph, Thanks for replying! On Wed, 17 May 2006, Christoph Lameter wrote: > On Wed, 17 May 2006, Steven Rostedt wrote: > > > My first attempt to fix this introduced another dereference, to allow > > for modules to allocate their own memory. This was quickly shot down, > > and for good reason, because dereferences kill performance, and don't > > play nice with large SMP systems that depend on per_cpu being fast. > > > I now place the per_cpu variables into VM, such that the pages are > > only allocated when needed. All the architecture needs to do is > > supply a VM address range, size for each CPU to use (note this > > implementation expects all the VM CPU areas to be together), and > > three functions to allow for allocating page tables at bootup. > > So now instead of an explicit indirection we use an implicit one > through the page tables for this. This happens during early boot which > requires additional page table functions? And it requires the use of an > additional TLB entry? I guess that the additional TLB pressure alone will > result in a performance drop of 3%? Ouch! > > See http://www.gelato.unsw.edu.au/archives/linux-ia64/0602/17311.html Thanks for the link. Hmm, my main goal is still to make the per_cpu more robust, so that the generic code is truely that, and the hacks are better managed. Would the TLB pressure on a normal desktop also cause the drop in performance? I haven't tried any benchmarks. Have any tests I can run on two kernels? I'm currently running my machine with the patches and I haven't noticed a difference. Although I'm not doing database work, I'm still compiling kernels. Reason I'm asking, is that I wonder if the whole VM idea is a waste, or is it only a problem on certain archs? Perhaps move the whole of percpu_boot_alloc into the arch, and let it do the allocation as is. Could perhaps use some arch specific register to calculate the entries. OK, now I'm just rambling. I don't know, have any other ideas on making this more robust? Or is this all in vain, and I should spend my evenings walking around this beautiful town of Karlsruhe ;) -- Steve
