Re: First set of OSDL Shared Mem scalability results, some wierdness ...

Kevin Brown <kevin@sysexperts.com> writes:
> Hmm...something just occurred to me about this.

> Would a hybrid approach be possible?  That is, use mmap() to handle
> reads, and use write() to handle writes?

Nope.  Have you read the specs regarding mmap-vs-stdio synchronization?
Basically it says that there are no guarantees whatsoever if you try
this.  The SUS text is a bit weaselly ("the application must ensure
correct synchronization") but the HPUX mmap man page, among others,
lays it on the line:

     It is also unspecified whether write references to a memory region
     mapped with MAP_SHARED are visible to processes reading the file and
     whether writes to a file are visible to processes that have mapped the
     modified portion of that file, except for the effect of msync().

It might work on particular OSes but I think depending on such behavior
would be folly...

            regards, tom lane