Re: pg_restore --multi-thread

Peter Eisentraut wrote:
> Andrew Dunstan wrote:
>> Cédric Villemain wrote:
>>>
>>>  -j [jobs], --jobs[=jobs]
>>>   Specifies  the  number  of jobs (pg_restore) to run 
>>> simultaneously. If the -j
>>> option is given without an argument, pg_restore will not limit the 
>>> number of
>>> jobs that can run simultaneously.
>
>> Quite apart from anything else, this description is almost 100% dead 
>> wrong.  The argument is not optional at all, and there is no 
>> unlimited parallelism. If you want to know how it actually works look 
>> at the dev docs.
>
> What I'm still missing here is a piece of documentation or a guideline 
> that says when a given number of threads/jobs/workers would be 
> appropriate.  For make -j, this is pretty clear: If you have N CPUs to 
> spare, use -j N.  For pg_restore, this is not made clear:  Is it the 
> number of CPUs on the client or the server or the number of disks on 
> the client or the server or perhaps a combination of this or something 
> else?

The short answer is that we don't know yet. There is anecdotal evidence 
that the number of CPUs on the server is a good place to start, but we 
should be honest enough to say that this is a new feature and we are 
still gathering information about its performance.  If you want to give 
some advice, then I think the best advice is to try a variety of 
settings to see what works best for you, and if you have a good set of 
figures report it back to us.

cheers

andrew