Messages in this thread | | | Date | Tue, 2 Aug 2022 10:08:47 +0100 | Subject | Re: [PATCH v3 00/17] Compress the pmu_event tables | From | John Garry <> |
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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
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