Re: Is Parallel Programming Hard, And, If So, What Can You Do About It?

On 5/23/2025 12:35 AM, Lawrence D'Oliveiro wrote:

On Fri, 23 May 2025 00:18:53 -0000 (UTC), Waldek Hebisch wrote:
>

It is pretty clear that due to drive mechanics track cache/buffer is
useful.

>
Only if you don’t take the statistics of real-world cache behaviour into
account.
>

However, the real question is about size: how big should it be.

>
It can never be big enough to make a difference.

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Sometimes, it is not the size of the cache that matters.
>
>
Say, for example, a typical cache configuration in my core:
   L1 D$: 32K, direct mapped
   L1 I$: 16K, direct mapped
   L2: 256K, direct mapped.
>
OK, so say I stick a 4K cache between the L1 and L2 caches.
Seems kinda useless just based on sizes.

Except: This small cache is 4-way set associative and so can absorb a
bunch of conflict misses, and notably reducing the number of cache
misses in the L2 cache. It can bring a performance benefit, despite
being small.

Why?