Fix the heuristics for L3 cache size for Arm64

[kunalspathak]

While L3 cache size is undetermined on Arm64 machines, if we don't get it, use a fixed heuristics based upon the core count.

1 ~ 4 : 4 MB
5 ~ 16 : 8 MB
17 ~ 64 : 16 MB
65+ : 32 MB

This led to removing the obscure heuristics we have currently that just multiplies the cache size by 3.

#if defined(HOST_ARM64)
// Bigger gen0 size helps arm64 targets
maxSize = maxTrueSize * 3;
#endif

[kunalspathak]

@Maoni0

[ghost]

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Issue Details

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While L3 cache size is undetermined on Arm64 machines, if we don't get it, use a fixed heuristics based upon the core count.

1 ~ 4 : 4 MB
5 ~ 16 : 8 MB
17 ~ 64 : 16 MB
65+ : 32 MB

Author:	kunalspathak
Assignees:	-
Labels:	area-GC-coreclr
Milestone:	-

[Maoni0]

Q (also applies to the old src) - what does sysconf return if there's no cache of that level? I presume it returns 0, not -1 (the doc says it returns -1 if there's an error).

[kunalspathak]

@dotnet/jit-contrib , @davidwrighton , @jkotas

[Maoni0]

LGTM! thanks @kunalspathak!

[janvorli]

One last nit - please feel free to do that later if you want to get this in before today's snap for preview 6.
Looking at the usages of this macro, I have realized it would be great to make the size a parameter of the macro too. From the usage sites, it is not obvious where it gets the size from (I have to read the macro definition to figure it out).

[kunalspathak]

Sure, will do it in follow-up PR

[kunalspathak]

#71209

[kunalspathak]

Linux/arm64 improvements:

• [Perf] Changes at 6/22/2022 11:10:17 PM perf-autofiling-issues#6510
• [Perf] Changes at 6/22/2022 5:33:38 PM perf-autofiling-issues#6509

windows/arm64 improvements:

• [Perf] Changes at 6/22/2022 1:08:18 PM perf-autofiling-issues#6499
• [Perf] Changes at 6/22/2022 11:10:17 PM perf-autofiling-issues#6501