Date: Wed, 16 Sep 2020 15:36:38 -0700 From: John-Mark Gurney <jmg@funkthat.com> To: Warner Losh <imp@bsdimp.com> Cc: Julian Grajkowski <julian.grajkowski@gmail.com>, freebsd-drivers@freebsd.org Subject: Re: getpid() performance Message-ID: <20200916223638.GR4213@funkthat.com> In-Reply-To: <CANCZdfrSa7pfOWRLr9jji3ePshTOxFMYmy3=fQAonOdJO9a50g@mail.gmail.com> References: <CAGQdsJhgAM4aHY46SaGDpXJGOdEzG6rD93WzG7RT9PWi_%2BwJrA@mail.gmail.com> <CANCZdfrSa7pfOWRLr9jji3ePshTOxFMYmy3=fQAonOdJO9a50g@mail.gmail.com>
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Warner Losh wrote this message on Wed, Sep 16, 2020 at 01:24 -0600:
> On Wed, Sep 16, 2020 at 1:15 AM Julian Grajkowski <
> julian.grajkowski@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.
if you really need to see if your process has forked (I assume that is
why you're calling getpid so frequently), you can mmap a page, and using
minherit's INHERIT_ZERO so that all the data in that page will be zero'd
on fork. You can then change your getpid check to something like:
pid_t *page_with_inherit_zero_set;
pid_t
my_getpid()
{
if (page_with_inherit_zero_set == NULL)
allocate_page_and_set_inherit_zero();
if (*page_with_inherit_zero_set == 0) {
*page_with_inherit_zero_set = getpid();
return *page_with_inherit_zero_set;
}
and you should see similar improvements.
Though this might allow you to move this logic to a better place in
your code.
--
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
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