Re: Profiling PostgreSQL

On Fri, May 23, 2014 at 10:25 AM, Dimitris Karampinas <dkarampin@gmail.com> wrote:

I want to bypass any disk bottleneck so I store all the data in ramfs (the purpose the project is to profile pg so I don't care for data loss if anything goes wrong).

Since my data are memory resident, I thought the size of the shared buffers wouldn't play much role, yet I have to admit that I saw difference in performance when modifying shared_buffers parameter.

In which direction?  If making shared_buffers larger improves things, that suggests that you have contention on the BufFreelistLock.  Increasing shared_buffers reduces buffer churn (assuming you increase it by enough) and so decreases that contention.

 

I use taskset to control the number of cores that PostgreSQL is deployed on.

It can be important what bits you set.  For example if you have 4 sockets, each one with a quadcore, you would probably maximize the consequences of spinlock contention by putting one process on each socket, rather than putting them all on the same socket.

 

Is there any parameter/variable in the system that is set dynamically and depends on the number of cores ?

The number of spins a spinlock goes through before sleeping, spins_per_delay, is determined dynamically based on how often a tight loop "pays off".  But I don't think this is very sensitive to the exact number of processors, just the difference between 1 and more than 1.