Michael Stone wrote:
> At one point someone complained about the ability to configure, e.g.,
> IRIX to allow memory overcommit. I worked on some large IRIX
> installations where full memory accounting would have required on the
> order of 100s of gigabytes of swap, due to large shared memory
> allocations.
These were mostly scientific and graphical apps where reliability took a back seat to performance and to program
complexity. They would allocate 100's of GB of swap space rather than taking the time to design proper data structures.
If the program crashed every week or two, no big deal -- just run it again. Overallocating memory is a valuable
techniquefor such applications.
But overallocating memory has no place in a server environment. When memory overcommittment is allowed, it is
impossibleto write a reliable application, because no matter how carefully and correctly you craft your code, someone
else'sprogram that leaks memory like Elmer Fudd's rowboat after his shotgun goes off, can kill your well-written
application.
Installing Postgres on such a system makes Postgres unreliable.
Tom Lane wrote:
> That might have been right when it was written (note the reference to a
> 2.2 Linux kernel), but it's 100% wrong now.
> [Setting /proc/sys/vm/overcommit_memory to] 0 is the default, not-safe
> setting.
I'm surprised that the Linux kernel people take such a uncritical view of reliability that they set, as *default*, a
featurethat makes Linux an unreliable platform for servers.
And speaking of SGI, this very issue was among the things that sank the company. As the low-end graphics cards ate
intotheir visualization market, they tried to become an Oracle Server platform. Their servers were *fast*. But they
crashed-- a lot. And memory-overcommit was one of the reasons. IRIX admins would brag that their systems only crashed
everycouple of weeks. I had HP and Sun systems that would run for years.
Craig
