Re: RFC: x86-64: Use fxsave/xsave/xsavec in _dl_runtime_resolve [BZ #21265]

[Florian Weimer <fw at deneb dot enyo dot de>]

* Florian Weimer:

> On 10/20/2017 03:21 PM, Florian Weimer wrote:
>> LD_DEBUG=statistics shows this:
>> 
>>        9506:
>>        9506:     runtime linker statistics:
>>        9506:       total startup time in dynamic loader: 19960074 cycles
>>        9506:                 time needed for relocation: 19105814 cycles 
>> (95.7%)
>>        9506:                      number of relocations: 87
>>        9506:           number of relocations from cache: 3
>>        9506:             number of relative relocations: 1226
>>        9506:                time needed to load objects: 701382 cycles 
>> (3.5%)
>>        9506:
>>        9506:     runtime linker statistics:
>>        9506:                final number of relocations: 781589
>>        9506:     final number of relocations from cache: 3
>> 
>> This is a main program which contains 1,500 function calls.  The 
>> functions are defined in a single DSO, and each function calls 520 other 
>> functions, giving a total number of 781,500 relocations from the test.
>> 
>> On my laptop (Intel(R) Core(TM) i7-4810MQ CPU @ 2.80GHz), I get this 
>> (ten runs, real time measured in seconds):
>> 
>>  > t.test(prev_laptop, after_laptop)
>> 
>>      Welch Two Sample t-test
>> 
>> data:  prev_laptop and after_laptop
>> t = -14.932, df = 18, p-value = 1.392e-11
>> alternative hypothesis: true difference in means is not equal to 0
>> 95 percent confidence interval:
>>   -0.05749145 -0.04330855
>> sample estimates:
>> mean of x mean of y
>>     0.2345    0.2849
>> 
>> So it's definitely not in the noise.  The penalty appears to be around 
>> 65ns per relocation.
>
> I did some more benchmarks.  Ryzen takes a bit of a hit (100ns or ~30%). 
>   Purley looks very good (35ns or ~5%).  The outlier is KNL, with 750ns 
> added per relocation (at lower clock rates admittedly) and ~50% longer 
> relocation times overall.  (The caveat is that this is lab hardware, 
> which may or may not match production silicon.)

I found another piece of perhaps interesting hardware, an Intel(R)
Core(TM) m7-6Y75.  The performance hit is around 3% or 16ns.  This is
a lower-power mobile CPU in a tablet.  So even there, the numbers are
good.