Cache line padding of Pool is incorrect

[ilnarb]

usual cache line size is 64 byte.
it seems sizeof (Pool) == 32
you need to write
char cache_line_padding[CACHE_LINE_SIZE - sizeof(freed) - sizeof(raw) - sizeof(elem_size)];

[ivan-tkatchev]

Why is that?
Why do you think that it's important that sizeof(Pool) be equal to the cache line size?

[ilnarb]

parts of two distinct Pool should not be placed into one cache line in order to prevent cache line contention. Current size of Pool == 32 bytes. In this case parts of two Pool always will be placed in a single cache line.
It's important that padding will align the next Pool object into different cache line. In case first Pool aligned to cache line and sizeof(Pool)== cache line size sizeof padding equal to CACHE_LINE_SIZE - sizeof(freed) - sizeof(raw) - sizeof(elem_size) will be enought. In other case, more padding is need.

[ilnarb]

to tell the truth, it will be better to aligning vs. apdding
attribute ((aligned(CACHE_LINE_SIZE)))

[ivan-tkatchev]

Yes, of course. However, a Pool corresponds to a pool of free blocks, where each block in the pool is of a fixed, predetermined size. (There are thus 1023 such Pools, one for each possible block size, up to a block size of 8k.) Contention should be a problem only if one thread only allocates memory blocks of one fixed size, with another thread also allocating only blocks of another fixed size; this should be unrealistic in practice. (All threads should, theoretically, be allocating blocks of all sizes, uniformly.)

[ilnarb]

in that case, how about eliminating the line (size_t cache_line_padding;) at all?
it doesn't pad, just confuses.

[ivan-tkatchev]

That's there to make Pool fit entirely into one cache line. I'm not really sure that brings any real benefit, but 32 is a nice and clean number, anyways. :)

[ivan-tkatchev]

That's there to make Pool fit entirely into one cache line. I'm not really sure that brings any real benefit, but 32 is a nice and clean number, anyways. :)

[ilnarb]

could i contact u tru icq? my num 8112260

[ivan-tkatchev]

I don't have an ICQ number, but you can contact me via jabber/gtalk at [email protected].

[ilnarb]

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