A couple of weeks ago, I went to a presentation about mutex
optimisations. It included measurements of the cost of a mutex lock
followed by unlock on Alpha, both with the usual
load-locked/store-conditional with memory barrier sequence, and with
the mutex code modified to use a load/store and no memory barriers
sequence. I don't remember the exact figures, but:
- The cost for the ll/sc version was >200 cycles on all of the EV4,
EV5, and EV6. The cost for the l/s version was <50 cycles on those
processors.
- The l/s version took less cycles on each successive generation of
processor, as you might expect. But the ll/sc version took more cycles
on EV6 than on EV5.
So compared to the competition, it doesn't seem that Intel is doing
too badly with its LOCK performance.
David Wragg
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