Re: [PATCH] x86: Add thresholds for "rep movsb/stosb" to tunables

[Carlos O'Donell via Libc-alpha <libc-alpha@sourceware.org>]

On 5/29/20 9:21 AM, H.J. Lu wrote:
> On Fri, May 29, 2020 at 6:13 AM Carlos O'Donell <carlos@redhat.com> wrote:
>>
>> On 5/23/20 12:37 AM, H.J. Lu via Libc-alpha wrote:
>>> There is no single threshold value which is good for all workloads.
>>> I don't think we should change REP_STOSB_THRESHOLD to 1MB.
>>> On the other hand, the fixed threshold isn't flexible.  Please try this
>>> patch to see if you can set the threshold for your specific workload.
>>
>> My request here is that the manual include a documentation of what the
>> minimums are for the tunable. Even an example reference of the minimum
>> value would be useful for the tunable e.g. On AVX512 systems this value
>> is X, on AVX systems this value is Y, on all other systems Z.
>>
> 
> The logic of thresholds are:
> 
>  /* NB: The REP MOVSB threshold must be greater than VEC_SIZE * 8.  */
>   unsigned int minimum_rep_movsb_threshold;
>   /* NB: The default REP MOVSB threshold is 2048 * (VEC_SIZE / 16).  */
>   unsigned int rep_movsb_threshold;
>   if (CPU_FEATURES_ARCH_P (cpu_features, AVX512F_Usable)
>       && !CPU_FEATURES_ARCH_P (cpu_features, Prefer_No_AVX512))
>     {
>       rep_movsb_threshold = 2048 * (64 / 16);
>       minimum_rep_movsb_threshold = 64 * 8;
>     }
>   else if (CPU_FEATURES_ARCH_P (cpu_features,
> AVX_Fast_Unaligned_Load))
>     {
>       rep_movsb_threshold = 2048 * (32 / 16);
>       minimum_rep_movsb_threshold = 32 * 8;
>     }
>   else
>     {
>       rep_movsb_threshold = 2048 * (16 / 16);
>       minimum_rep_movsb_threshold = 16 * 8;
>     }
>   if (cpu_features->rep_movsb_threshold > minimum_rep_movsb_threshold)
>     __x86_rep_movsb_threshold = cpu_features->rep_movsb_threshold;
>   else
>     __x86_rep_movsb_threshold = rep_movsb_threshold;
> 
> We can't simply say AVX512 machines will use ZMM and AVX machines
> will use YMM.  It depends on other factors which are invisible to users.
> Can you suggest some paragraph for libc manual?

We must tell the users the lower limit, so they can avoid having their
settings ignored.

If we can't tell them the lower limit in the manual, then we must add
a way to print it.

Augment the libc.so.6 main() entry point to print all tunables with
a --list-tunables option and print the limit? Then in the manual just
say you have to look it up?

-- 
Cheers,
Carlos.