On 10-Jan-19 9:01 PM, Gage Eads wrote:
For 64-bit architectures, doubling the head and tail index widths greatly increases the time it takes for them to wrap-around (with current CPU speeds, it won't happen within the author's lifetime). This is important in avoiding the ABA problem -- in which a thread mistakes reading the same tail index in two accesses to mean that the ring was not modified in the intervening time -- in the upcoming non-blocking ring implementation. Using a 64-bit index makes the possibility of this occurring effectively zero.I tested this commit's performance impact with an x86_64 build on a dual-socket Xeon E5-2699 v4 using ring_perf_autotest, and the change made no significant difference -- the few differences appear to be system noise. (The test ran on isolcpus cores using a tickless scheduler, but some variation was stll observed.) Each test was run three times and the results were averaged: | 64b head/tail cycle cost minus Test | 32b head/tail cycle cost ------------------------------------------------------------------ SP/SC single enq/dequeue | 0.33 MP/MC single enq/dequeue | 0.00 SP/SC burst enq/dequeue (size 8) | 0.00 MP/MC burst enq/dequeue (size 8) | 1.00 SP/SC burst enq/dequeue (size 32) | 0.00 MP/MC burst enq/dequeue (size 32) | -1.00 SC empty dequeue | 0.01 MC empty dequeue | 0.00 Single lcore: SP/SC bulk enq/dequeue (size 8) | -0.36 MP/MC bulk enq/dequeue (size 8) | 0.99 SP/SC bulk enq/dequeue (size 32) | -0.40 MP/MC bulk enq/dequeue (size 32) | -0.57 Two physical cores: SP/SC bulk enq/dequeue (size 8) | -0.49 MP/MC bulk enq/dequeue (size 8) | 0.19 SP/SC bulk enq/dequeue (size 32) | -0.28 MP/MC bulk enq/dequeue (size 32) | -0.62 Two NUMA nodes: SP/SC bulk enq/dequeue (size 8) | 3.25 MP/MC bulk enq/dequeue (size 8) | 1.87 SP/SC bulk enq/dequeue (size 32) | -0.44 MP/MC bulk enq/dequeue (size 32) | -1.10 An earlier version of this patch changed the head and tail indexes to uint64_t, but that caused a performance drop on 32-bit builds. With uintptr_t, no performance difference is observed on an i686 build. Signed-off-by: Gage Eads <[email protected]> ---
You're breaking the ABI - version bump for affected libraries is needed. -- Thanks, Anatoly
