On Wed, 9 May 2007 15:16:11 -0700 "Yu, Fenghua" <[EMAIL PROTECTED]> wrote:
> > >hm, DEFINE_PER_CPU_SHARED_CACHELINE_ALIGNED is a bit of a mouthful. > > >I wonder if we can improve things here so that we use the > runtime-detected > >cacheline size rather than the compile-time size. I guess not, given > that > >the offsets into the percpu area are calculated at build-time. > > >Did you work out how much space this change will actually save? It > >should be available by suitable crunching on the nm and objdump output. > > Depending on how data fields are arranged by linker, the patches could > save or waste per_cpu size. Below is data I got. > > > Case 1: On linux-2.6.21-rc7-mm2 with defconfig build. > Case 2: On linux-2.6.21-rc7-mm2 plus the patches in this thread with > defconfig build. > Case 3: On linux-2.6.21-rc7-mm2 with defconfig with VSMP=y build. > Case 4: On linux-2.6.21-rc7-mm2 plus the patches in this thread with > defconfig with VSMP=y build. > > Please note that on x86/x86-64, per_cpu_init_tss is placed in the first > place in per_cpu section in Case 1 and 3. And thus there is no padding > waste for per_cpu_init_tss in Case 1 and 3. > > On X86: > Case 1: Size of per_cpu section is: 0x7768 > Case 2: Size of per_cpu section is: 0x790c > The patches waste 0x1a4 bytes. > > per_cpu__init_tss, per_cpu__irq_stat, and per_cpu__runqueues are moved > to shared_cacheline_aligned section. > > On X86-64: > Case 1: Size of per_cpu section is: 0x72d0 > Case 2: Size of per_cpu section is: 0x6540 > The patches save 0xd90 bytes. > > Case 3: Size of per_cpu section is: 0x72d0 > Case 4: Size of per_cpu section is: 0x8340 > The patches waste 0x1070 bytes. > > Shall we not use shared_cacheline_aligned section for VSMP case? The > waste of cache eventually may offset the potential gain of alignment. > > Probably need to set up a cache line size threshold: if L1 cache line > size is bigger than a number CACHELINE_ALIGN_SHRESHOLD, don't do > cacheline alignment. > > per_cpu__init_tss and per_cpu__runqueues are moved to > shared_cacheline_aligned section. > > On ia64: > Case 1: Size of per_cpu section is: 0x8370 > Case 2: Size of per_cpu section is: 0x7fc0 > The patches save 0x3b0 bytes. > > per_cpu_ipi_operation and per_cpu_runqueues are moved to > shared_cacheline_aligned section erm, it's not obviosu from all this that the patches are worth proceeding with, are they? - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
