Re: performance tuning: shared_buffers, sort_mem; swap

>>>>> "Tom" == Tom Lane <tgl@sss.pgh.pa.us> writes:

Tom> David Gilbert <dgilbert@velocet.ca> writes:
>> I don't often ask a question, but it's been bugging me for some
>> time: is there any reason why PostgreSQL must use SysV shared
>> memory?  Coming from the BSD camp, I've often pondered why it
>> doesn't use BSD-style shared memory (which is often easier to
>> allocate in the BSD world).

Tom> Well, I must say this is the first time I've heard of "BSD-style
Tom> shared memory".  What are the syscalls?  How portable is it?
Tom> Does it have the semantics we need (specifically, the ability to
Tom> associate an ID with a shmem segment, and the ability to discover
Tom> whether any other processes are attached to an existing shmem
Tom> segment)?

The canonical example is to mmap() a file multiply.  BSD will
guarentee this is shared memory.  Of course, to address the concern
that you really don't need a file, the MAP_ANON flag can be given
(meaning that there is no file).

With anonymous memory, you pass the file descriptor through your
fork()'s.  With non-anonymous memory, you can mmap() it multiply from
different processes and get the same effect.

With a glance at the man page, you'd have MAP_SHARED | MAP_ANON |
MAP_NOSYNC | MAP_HASSEMAPHORE.

One bonus of bsd shared memory (when anonymous) is that it dies with
the process ... even in the worst case.

Dave.

--
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|David Gilbert, Velocet Communications.       | Two things can only be     |
|Mail:       dgilbert@velocet.net             |  equal if and only if they |
|http://daveg.ca                              |   are precisely opposite.  |
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