On Mon, 2004-10-18 at 04:13, Tom Lane wrote: > Do you have any actual evidence for that opinion? ISTM this is > dependent on a large set of assumptions about the CPU's bus behavior, > boiling down to the conclusion that an extra conditional branch is > cheaper than a locked bus cycle.
I think the conditional branch is effectively free: we're spinning in a busy-wait loop, so we're effectively throwing away cycles until the lock is free anyway. The important things are: (a) we interfere as little as possible with concurrent system activity while spinning (b) we notice promptly that the lock is free. ISTM that doing a non-locking test before the locking test achieves those two conditions. > (For that matter, I'm not that thrilled about it even for the Intel > chips, since the extra test is certainly 100% wasted cycles on any > single-CPU machine, or indeed anywhere that the lock is not contended > for.) Yes, but the proper fix for this is not to spin at all on UP machines; I have a grungy patch to do this that I will clean up and send in for 8.1. > > if (*lock == 0) > > TAS(lock); > > This will certainly not work on HPPA, and it really shouldn't assume > that zero is the non-locked state, and what happened to testing the > TAS result? Erm, perhaps I should have emphasized that that was pseudo-code :) A more realistic implementation would be: #define TAS_INNER_LOOP(lock) (S_LOCK_FREE(lock) ? TAS(lock) : 1) -Neil ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
