* Ingo Molnar (mingo@xxxxxxx) wrote: > > * Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > If it's faster, this becomes a legit (albeit complex) > > > micro-optimization in a _very_ hot codepath. > > > > I don't think it's all that hot. It's not like it's the return to > > user mode. > > Well i guess it depends. For server apps it is true - syscalls are a > lot more dominant, MMs are long-running so any startup cost gets > amortized and pagefaults are avoided. > > For something like a kernel build we have 7 times as many pagefaults > as syscalls: > > aldebaran:~/linux/linux> perf stat -- make -j32 >/dev/null > [...] > Performance counter stats for 'make -j32': > > 1444281.076741 task-clock-msecs # 14.429 CPUs > 219991 context-switches # 0.000 M/sec > 18335 CPU-migrations # 0.000 M/sec > 38465628 page-faults # 0.027 M/sec > 4374762924204 cycles # 3029.025 M/sec > 2645979309823 instructions # 0.605 IPC > 42398991227 cache-references # 29.356 M/sec > 4371920878 cache-misses # 3.027 M/sec > > 100.097787566 seconds time elapsed. > > So we have 38465628 page-faults, or one every 68788 instructions, > one every 113731 cycles. > > 10 cycles saved in the page fault costs means 0.01% performance win > - or about 10 milliseconds shaven off the kernel build time. > > 100 cycles saved (which is impossible really in the entry/exit path) > would mean 0.1% win. > > 5653639 syscalls (according to strace -c) - which is a factor of 6.8 > lower. Same goes for shell scripts or most of the clicking we do on > a GUI. > > It's not a big factor for sure. > > Btw., the biggest pagefault cost is in the fault handling itself > (the page clearing): > > 4.14% [k] do_page_fault > 1.20% [k] sys_write > 1.10% [k] sys_open > 0.63% [k] sys_exit_group > 0.48% [k] smp_apic_timer_interrupt > 0.37% [k] sys_read > 0.37% [k] sys_execve > 0.20% [k] sys_mmap > 0.18% [k] sys_close > 0.14% [k] sys_munmap > 0.13% [k] sys_poll > 0.09% [k] sys_newstat > 0.07% [k] sys_clone > 0.06% [k] sys_newfstat > > it totals to 4.14% of the total cost (user-space cycles included) of > a kernel build, on a Nehalem box. > Yes, page faults caused by COW of short-lived processes and faulting-in execs and libraries account for an insane portion of the build time. :-/ Mathieu > Ingo -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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