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From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-08-26 20:11:01

Howard Hinnant:

> If there are two threads sharing a processor, and one thread is
> accessing M1 amount of memory and the other thread is accessing M2
> amount of memory, and M1 + M2 is less than the L1 cache size, isn't
> that faster than if both M1 and M2 are less than the L1 cache size but
> M1 + M2 is larger than the L1 cache size? I.e. can larger memory use
> (or more scattered memory use) on one thread make cache misses more
> likely on another thread?

This is possible in principle (*), but I'm not sure that it can happen in
our particular case. At condition::wait thread 1 enters the kernel and is
blocked, and thread 2 is awakened. Only a few additional cache lines (one
for the "checked release" code, more when using a hash map) are touched by
the check in wait; it should be impossible to measure a difference. The
checks only occur once per timeslice. Even in the case of a spurious
immediate wakeup, when wait will be retried, there are no additional cache
line accesses since the memory touched by the check is already in cache.

(*) M1+M2 < L1.size seems a bit improbable though; this means that two
timeslices worth of (possibly unrelated) code can stay entirely within the
L1 cache. L2.size seems a more likely threshold.

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