getpid() performance

[Warner Losh]

On Wed, Sep 16, 2020 at 1:15 AM Julian Grajkowski <
julian.grajkowski at gmail.com> wrote:

> Hi,
>
> I am working on a contiguous memory allocator which frequently calls
> getpid() in user space and I have noticed very poor performance of this
> function call. I measured this call performance using following code:
>
> inline uint64_t rdtsc_start(void)
> {
>     uint32_t cycles_high;
>     uint32_t cycles_low;
>
>     asm volatile("lfence\n\t"
>                  "rdtscp\n\t"
>                  "mov %%edx, %0\n\t"
>                  "mov %%eax, %1\n\t"
>                  : "=r" (cycles_high), "=r" (cycles_low)
>                  : : "%rax", "%rdx", "%rcx");
>
>     return (((uint64_t)cycles_high << 32) | cycles_low);
> }
>
>
> inline uint64_t rdtsc_end(void)
> {
>     uint32_t cycles_high;
>     uint32_t cycles_low;
>
>     asm volatile("rdtscp\n\t"
>                  "mov %%edx, %0\n\t"
>                  "mov %%eax, %1\n\t"
>                  "lfence\n\t"
>                  : "=r" (cycles_high), "=r" (cycles_low)
>                  : : "%rax", "%rdx", "%rcx");
>
>     return (((uint64_t)cycles_high << 32) | cycles_low);
> }
>
> This way I measured ~320 cycles used for getpid() on FreeBSD 12.1. For
> comparison, in Linux (CentOS 7) this call uses ~10 cycles. I am aware that
> this should not be compared directly. as these are different systems, but
> such a big difference in performance is an issue for me, as getpid() is
> called very often in my code.
>
> Is such a poor performance of getpid() a known problem and is it possible
> that this might be improved in future releases?
>

glibc optimizes getpid() system call so it only calls it once and returns a
cached value (which is in line with 10 cycles, there's no way you can
save/restore state in 10 cycles, let alone do a dispatch). FreeBSD doesn't.

Warner


> Measurements were done on the same mahcine with following setup:
>
> CPU: Intel(R) Atom(TM) CPU C3958 @ 2.00GHz (2000.06-MHz K8-class CPU)
> FreeBSD/SMP: Multiprocessor System Detected: 16 CPUs
>
> 8GB RAM (2x4GB):
>         Type: DDR4
>         Type Detail: Synchronous Unbuffered (Unregistered)
>         Speed: 2400 MT/s
>
> Thank you very much in advance for any help.
>
> Kind regards,
> Julian
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