On Tue, Apr 5, 2011 at 2:21 PM, Mischa Sandberg <mischa.sandb...@sophos.com> wrote: > Came across the following in a paper from Oct 2010. Was wondering is this is > old news I missed in this group. > > http://pdos.csail.mit.edu/papers/linux:osdi10.pdf > > about Linux optimization on multi-core CPU’s. > > The group at MIT were exploring how some Linux apps were scaling up --- > sometimes badly, mostly due to hidden contention over cache-line consistency > across the cores’ caches. > > In a nutshell: if an app, or the system calls it uses, tries to modify > anything in a cache line (32-64 byte slice of memory) that another core is > using, there’s a lot of fumbling in the dark to make sure there is no > conflict. When I saw PostgreSQL named in the abstract, I thought, “Aha! > Contention over shm”. Not so. Skip to page 11 (section 5.5) for most of the > PG specifics.
Someone posted this before, but unfortunately making this really work in PG is more of a research project than something we can just go do. I made a stab at writing a spinlock-free version of the LWLock code a few months ago (which is one of the things they did in the paper) and I wasn't able to show a lick of benefit. Part of that may be because I didn't have access to anything bigger than an 8-core box, but it's also because these things are fairly workload-dependent. In the test cases I tried I kept bottlenecking on WALInsertLock or, on read-only workloads, the lock manager partition lock for whichever table I was hitting, and the changes they made don't address those bottlenecks. As they write - regarding their benchmark - "This workload is intended to minimize application-level contention within PostgreSQL in order to maximize the stress PostgreSQL places on the kernel." -- i.e. PostgreSQL wasn't really the thing they were trying to stress. It's interesting stuff - I'm just not sure how much near-term practical benefit we can get out of it. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers