[dpdk-dev] [PATCH 1/6] ring: change head and tail to pointer-width size

[Burakov, Anatoly]

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 <gage.eads at intel.com>
> ---

You're breaking the ABI - version bump for affected libraries is needed.

-- 
Thanks,
Anatoly