Re: [wwwdocs] gcc-12/changes.html (GCN): >1 workers per gang

[Tobias Burnus <tobias@codesourcery.com>]

Hi all,

On 09.08.21 20:53, Gerald Pfeifer wrote:

> (Is "CU" a sufficiently established term, or might it make sense
> to spell it out?)
I don't know – but we could use "per compute unit (CU)".

On 09.08.21 16:27, Thomas Schwinge wrote:
> On 2021-08-09T15:55:07+0200, Tobias Burnus<tobias@codesourcery.com>  wrote:
>> +++ b/htdocs/gcc-12/changes.html
>> +  <li>When used as OpenACC device: the limitation of 1 worker per gang, 2 gangs
>> +      per CU has been lifted; now up to 16 workers per gang and 40 gangs per CU
>> +      are supported. (Except that the hardware limit of 40 workers total may
>> +      not be exceeded.)</li>
> I haven't changed anything related to a "limitation of [...] 2 gangs per
> CU has been lifted".  Maybe that has already been done earlier, maybe
> that still has to be done?  I don't know -- Julian?

Looking at the current code, it has:
   if (dims[0] == 0) dims[0] = get_cu_count (kernel->agent); /* Gangs.  */

Thus at least when nothing else has been specified, it uses #CUs of gangs,
running on #CUs CUs, i.e. 1 gang per CU.

[OG11 – but not mainline] What's needed is something like:
       dims[0] = get_cu_count (kernel->agent) * (32 / dims[1]);
which I see in OG11 – oddly, I also see there code like:
           def->gdims[0] = get_cu_count (agent); // * (40 / gcn_threads);

In other words:  For gangs > #CUs or >1 gang per CU, the following patch
is needed:
   [OG11] https://gcc.gnu.org/g:4dcd1e1f4e6b451aac44f919b8eb3ac49292b308
   [email] https://gcc.gnu.org/pipermail/gcc-patches/2020-July/550102.html
      "not suitable for mainline until the multiple-worker support is merged there"

@Andrew + @Julian:  Do you intent to commit it relatively soon?
Regarding the wwwdocs patch, I can hold off until that commit or reword
it to only cover the workers part.

Thanks Thomas & Gerald for the comments!

Tobias

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