Re: [PATCH] Save and restore xmm0-xmm7 in _dl_runtime_resolve

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Jul 27, 2015 at 6:26 AM, OndÅej BÃlka <neleai@seznam.cz> wrote:
> On Mon, Jul 27, 2015 at 06:14:07AM -0700, H.J. Lu wrote:
>> On Mon, Jul 27, 2015 at 3:10 AM, OndÅej BÃlka <neleai@seznam.cz> wrote:
>> > On Sun, Jul 26, 2015 at 12:38:20PM -0700, H.J. Lu wrote:
>> >> On Sun, Jul 26, 2015 at 6:16 AM, OndÅej BÃlka <neleai@seznam.cz> wrote:
>> >> > On Sat, Jul 11, 2015 at 04:50:02PM -0700, H.J. Lu wrote:
>> >> >> On Sat, Jul 11, 2015 at 01:27:42PM -0700, H.J. Lu wrote:
>> >> >> > On Sat, Jul 11, 2015 at 12:46:54PM +0200, OndÅej BÃlka wrote:
>> >> >> > > On Thu, Jul 09, 2015 at 09:07:24AM -0700, H.J. Lu wrote:
>> >> >> > > > On Thu, Jul 9, 2015 at 7:28 AM, OndÅej BÃlka <neleai@seznam.cz> wrote:
>> >> >> > > > > On Thu, Jul 09, 2015 at 07:12:24AM -0700, H.J. Lu wrote:
>> >> >> > > > >> On Thu, Jul 9, 2015 at 6:37 AM, Zamyatin, Igor <igor.zamyatin@intel.com> wrote:
>> >> >> > > > >> >> On Wed, Jul 8, 2015 at 8:56 AM, Zamyatin, Igor <igor.zamyatin@intel.com>
>> >> >> > > > >> >> wrote:
>> >> >> > > > >> >> > Fixed in the attached patch
>> >> >> > > > >> >> >
>> >> >> > > > >> >>
>> >> >> > > > >> >> I fixed some typos and updated sysdeps/i386/configure for
>> >> >> > > > >> >> HAVE_MPX_SUPPORT.  Please verify both with HAVE_MPX_SUPPORT and
>> >> >> > > > >> >> without on i386 and x86-64.
>> >> >> > > > >> >
>> >> >> > > > >> > Done, all works fine
>> >> >> > > > >> >
>> >> >> > > > >>
>> >> >> > > > >> I checked it in for you.
>> >> >> > > > >>
>> >> >> > > > > These are nice but you could have same problem with lazy tls allocation.
>> >> >> > > > > I wrote patch to merge trampolines, which now conflicts. Could you write
>> >> >> > > > > similar patch to solve that? Original purpose was to always save xmm
>> >> >> > > > > registers so we could use sse2 routines which speeds up lookup time.
>> >> >> > > >
>> >> >> > > > So we will preserve only xmm0 to xmm7 in _dl_runtime_resolve? How
>> >> >> > > > much gain it will give us?
>> >> >> > > >
>> >> >> > > I couldn't measure that without patch. Gain now would be big as we now
>> >> >> > > use byte-by-byte loop to check symbol name which is slow, especially
>> >> >> > > with c++ name mangling. Would be following benchmark good to measure
>> >> >> > > speedup or do I need to measure startup time which is bit harder?
>> >> >> > >
>> >> >> >
>> >> >> > Please try this.
>> >> >> >
>> >> >>
>> >> >> We have to use movups instead of movaps due to
>> >> >>
>> >> >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58066
>> >> >>
>> >> >>
>> >> > Thanks, this looks promising.
>> >> >
>> >> > I think how to do definite benchmark, Now I have evidence that its
>> >> > likely improvement but not definite.
>> >> >
>> >> > I found that benchmark that i intended causes too much noise and I
>> >> > didn't get useful from that yet. It was creating 1000 functions in
>> >> > library and calling them from main where performance between runs vary
>> >> > by factor of 3 for same implementation.
>> >> >
>> >> > I have indirect evidence. With attached patch to use sse2 routines I
>> >> > decreased startup time of running binaries when you run "make bench"
>> >> > by ~6000 cycles and dlopen time by 4% on haswell and ivy bridge.
>> >> >
>> >> > See results on haswell of
>> >> >
>> >> > LD_DEBUG=statistics make bench &> old_rtld
>> >> >
>> >> > that are large so you could browse these here
>> >> >
>> >> > http://kam.mff.cuni.cz/~ondra/old_rtld
>> >> > http://kam.mff.cuni.cz/~ondra/new_rtld
>> >> >
>> >> > For dlopen benchmark I measure ten times performance of
>> >> > dlopen(RTLD_DEFAULT,"memcpy");
>> >> > dlopen(RTLD_DEFAULT,"strlen");
>> >> >
>> >> > Without patch I get
>> >> >  624.49  559.58  556.6 556.04  558.42  557.86  559.46  555.17  556.93  555.32
>> >> > and with patch
>> >> >   604.71  536.74  536.08  535.78  534.11  533.67  534.8 534.8 533.46 536.08
>> >> >
>> >> > I attached vip patches, I didn't change memcpy yet.
>> >> >
>> >> > So if you have idea how directly measure fixup change it would be
>> >> > welcome.
>> >> >
>> >>
>> >> There is a potential performance issue.  This won't change parameters
>> >> passed in S256-bit/512-bit vector registers because SSE load will only
>> >> update the lower 128 bits of 256-bit/512-bit vector registers while
>> >> preserving the upper bits.  But these SSE load operations may not be
>> >> fast on all current and future processors.  To load the entire
>> >> 256-bit/512-bit vector registers, we need to check CPU feature in
>> >> each symbol lookup.  On the other hand, we can compile x86-64 ld.so
>> >> with -msse2.  I don't know what the final performance impact is.
>> >>
>> > Yes, these should be saved due problems with modes. There could be
>> > problem that saving these takes longer. You don't need
>> > check cpu features on each call.
>> > Make _dl_runtime_resolve a function pointer and on
>> > startup initialize it to correct variant.
>>
>> One more indirect call.
>>
> no, my proposal is different, we could do this:
>
> void *_dl_runtime_resolve;
> int startup()
> {
>   if (has_avx())
>     _dl_runtime_resolve = _dl_runtime_resolve_avx;
>   else
>     _dl_runtime_resolve = _dl_runtime_resolve_sse;
> }
>
> Then we will assign correct variant.

Yes, this may work for both _dl_runtime_profile and
 _dl_runtime_resolve.  I will see what I can do.


-- 
H.J.