Re: An unlikely() experiment
| От | David Rowley |
|---|---|
| Тема | Re: An unlikely() experiment |
| Дата | |
| Msg-id | CAKJS1f_XBgcRP0tuAWzS1OF27P3V0k-bLpm=X3_+ey3KwF1giA@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: An unlikely() experiment (Andres Freund <andres@anarazel.de>) |
| Ответы |
Re: An unlikely() experiment
|
| Список | pgsql-hackers |
On 20 December 2015 at 03:06, Andres Freund <andres@anarazel.de> wrote:
On 2015-12-20 02:49:13 +1300, David Rowley wrote:
> So I choose to ignore that, and give
> it a try anyway... elog(ERROR) for example, surely that branch should
> always be in a cold path? ... ereport() too perhaps?
Hm, not exactly what you were thinking of, but it'd definitely be
interesting to add unlikely annotations to debug elogs, in some
automated manner (__builtin_choose_expr?). I've previously hacked
elog/ereport to only support >= ERROR, and that resulted in a speedup,
removing a log of elog(DEBUG*) triggered jumps.
Good thinking. It would be interesting to see benchmarks with the hacked version of elog()
To make sure I understand correctly: Before doing that you manually
added the errcheck()'s manually? At each elog(ERROR) callsite?
Yeah manually at most call sites. Although I did nothing in cases like;
if (somethinggood)
dosomethinggood();
else
elog(ERROR, ...);
Quite possibly in that case we could use likely(), but there's also cases where elog() is in the default: in a switch(), or contained in an else in an if/else if/else block.
I also left out errcheck() in places where the condition called a function with some side affect. I did this as I also wanted to test a hard coded false condition to see how much overhead remained. I did nothing with ereport().
One way to do this would be to add elog_on() / ereport_on() macros,
directly containing the error message. Like
#define elog_on(cond, elevel, ...) \
do { \
if (unlikely(cond)) \
{ \
elog(elevel, __VA_ARGS__) \
} \
} while(0)
I've attached the patch this time, it should apply cleanly to bbbd807. I'd would be good to see some other performance tests on some other hardware.
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