Linux kernel impact on PostgreSQL performance

Hi,

I'm the chair for Linux Storage, Filesystem and Memory Management Summit 2014
(LSF/MM). A CFP was sent out last month (https://lwn.net/Articles/575681/)
that you may have seen already.

In recent years we have had at least one topic that was shared between
all three tracks that was lead by a person outside of the usual kernel
development community. I am checking if the PostgreSQL community
would be willing to volunteer someone to lead a topic discussing
PostgreSQL performance with recent kernels or to highlight regressions
or future developments you feel are potentially a problem. With luck
someone suitable is already travelling to the collaboration summit
(http://events.linuxfoundation.org/events/collaboration-summit) and it
would not be too inconvenient to drop in for LSF/MM as well.

There are two reasons why I'm suggesting this. First, PostgreSQL was the
basis of a test used to highlight a scheduler problem around kernel 3.6
but otherwise in my experience it is rare that PostgreSQL is part of a
bug report. I am skeptical this particular bug report was a typical use
case for PostgreSQL (pgbench, read-only, many threads, very small in-memory
database). I wonder why reports related to PostgreSQL are not more common.
One assumption would be that PostgreSQL is perfectly happy with the current
kernel behaviour in which case our discussion here is done.

This brings me to the second reason -- there is evidence
that the PostgreSQL community is not happy with the current
direction of kernel development. The most obvious example is this thread
http://postgresql.1045698.n5.nabble.com/Why-we-are-going-to-have-to-go-DirectIO-td5781471.html
but I suspect there are others. The thread alleges that the kernel community
are in the business of pushing hackish changes into the IO stack without
much thought or testing although the linked article describes a VM and not
a storage problem. I'm not here to debate the kernels regression testing
or development methodology but LSF/MM is one place where a large number
of people involved with the IO layers will be attending. If you have a
concrete complaint then here is a soap box.

Does the PostgreSQL community have a problem with recent kernels,
particularly with respect to the storage, filesystem or memory management
layers? If yes, do you have some data that can highlight this and can you
volunteer someone to represent your interests to the kernel community? Are
current developments in the IO layer counter to the PostgreSQL requirements?
If so, what developments, why are they a problem, do you have a suggested
alternative or some idea of what we should watch out for? The track topic
would be up to you but just as a hint, we'd need something a lot more
concrete than "you should test more".

--
Mel Gorman
SUSE Labs