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MEDIUM: ring: improve speed in the queue waiting loop on x86_64

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x86_64 doesn't have a native atomic FETCH_OR(), it's implemented using
a CAS, which will always cause a write cycle. Here we know we can just
wait as long as the lock bit is held so better loop on a load, and only
attempt the CAS on success. This requires a tiny ifdef and brings nice
benefits. This brings the performance back from 3.33M to 3.75M at 24C48T
while doing no change at 3C6T.
This commit is contained in:
Willy Tarreau 2024-03-17 10:20:56 +01:00
parent 30a659c355
commit 573bbbe127
1 changed files with 9 additions and 5 deletions
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8
src/ring.c
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@ -281,11 +281,15 @@ ssize_t ring_write(struct ring *ring, size_t maxlen, const struct ist pfx[], siz
if (next_cell != &cell)
goto wait_for_flush; // FIXME: another thread arrived, we should go to wait now
__ha_cpu_relax_for_read();
#if defined(__x86_64__)
/* x86 prefers a read first */
if (!(HA_ATOMIC_LOAD(tail_ptr) & RING_TAIL_LOCK))
#endif
{
tail_ofs = HA_ATOMIC_FETCH_OR(tail_ptr, RING_TAIL_LOCK);
if (!(tail_ofs & RING_TAIL_LOCK))
break;
}
__ha_cpu_relax_for_read();
}
/* OK the queue is locked, let's attempt to get the tail lock */