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.