Re: [RFC PATCH v7 7/7] Restartable sequences: self-tests

[Mathieu Desnoyers <mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>]

----- On Jul 24, 2016, at 2:01 PM, Dave Watson davejwatson-b10kYP2dOMg@public.gmane.org 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