Messages in this thread | | | Date | Mon, 25 Jul 2016 16:43:29 +0000 (UTC) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH v7 7/7] Restartable sequences: self-tests |
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----- On Jul 24, 2016, at 2:01 PM, Dave Watson davejwatson@fb.com wrote:
>>> +static inline __attribute__((always_inline)) >>> +bool rseq_finish(struct rseq_lock *rlock, >>> + intptr_t *p, intptr_t to_write, >>> + struct rseq_state start_value) > >>> This ABI looks like it will work fine for our use case. I don't think it >>> has been mentioned yet, but we may still need multiple asm blocks >>> for differing numbers of writes. For example, an array-based freelist push: > >>> void push(void *obj) { >>> if (index < maxlen) { >>> freelist[index++] = obj; >>> } >>> } > >>> would be more efficiently implemented with a two-write rseq_finish: > >>> rseq_finish2(&freelist[index], obj, // first write >>> &index, index + 1, // second write >>> ...); > >> Would pairing one rseq_start with two rseq_finish do the trick >> there ? > > Yes, two rseq_finish works, as long as the extra rseq management overhead > is not substantial.
The different is actually not negligible. On x86-64 Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz: (counter increment benchmark (single-thread))
* Single store per increment: 3.6 ns * Two rseq_finish() per increment: 5.2 ns * rseq_finish2() with two mov instructions per rseq_finish2(): 4.0 ns
And I expect the difference to be even larger on non-x86 architectures.
I'll try to figure out a way to do rseq_finish() and rseq_finish2() without duplicating the code. Perhaps macros will be helpful there.
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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