Re: [PATCH v3 00/17] Compress the pmu_event tables

[John Garry <john.garry@huawei.com>]

On 29/07/2022 18:27, Ian Rogers wrote:
>> This implementation would require core pmu.c to be changed, but there is
>> ways that this could be done without needing to change core pmu.c
>>
>> Thanks,
>> John
> Thanks John!
> 
> You are right about broadwell, it is an extreme case of sharing. IIRC
> BDX is the server core/uncore events, BDW is the consumer core/uncore
> and BDW-DE is consumer core with server uncore - so the sharing is
> inherent in this. Metrics become interesting as they may mix core and
> uncore, but I'll ignore that here.
> 
> In the old code every event needs 15 char*s, with 64-bit that is 15*8
> bytes per entry with 741 core and 23 uncore entries for BDW, and 372
> core and 1284 uncore for BDX. I expect the strings themselves will be
> shared by the C compiler, and so I just focus on the pointer sizes.
> With the new code every event is just 1 32-bit int. So for BDW we go
> from 15*8*(741+23)=91,680 to 4*(741+23)=3056, BDX is
> 15*8*(372+1284)=198720 to 4*(372+1284)=6624. This means we've gone
> from 290,400bytes to 9,680bytes for BDW and BDX. BDW-DE goes from
> 243,000bytes to 8,100bytes -


> we can ignore the costs of the strings as
> they should be fully shared, especially for BDW-DE.

Are you sure about this? I did not think that the compiler would have 
the intelligence to try to share strings. That is the basis of the size 
optimisation which I was proposing (that the compiler would not share 
strings).

> 
> If we added some kind of table sharing, so BDW-DE was core from BDW
> and uncore from BDX and the tables shared, then in the old code you
> could save nearly 0.25MB but with the new code the saving is only
> around 8KB. I think we can go after that 8KB but it is less urgent
> after this change which gets 96% of the benefit. We have 72
> architectures for jevents at the moment and so I'd ball park (assuming
> they all saved as much as BDW-DE) the max saving as about 0.5MB, which
> is 12% of what is saved here.
> 
> Longer term I'd like to make the pmu-events.c logic look closer to the
> sysfs API. Perhaps we can unify the uncore events for BDX and BDW-DE
> with some notion of a common PMU, or PMUs with common events and
> tables, and automate deduction of this. It also isn't clear to me the
> advantage of storing the metrics inside the events, separate tables
> feel cleaner. Anyway, there's lots of follow up.

Thanks,
John