[ Send to hackers]
> I'd be willing to consider using mmap as a compile-time option if it
> can be shown to be a substantial performance win where it's available.
> (I suspect that's a very big "if".) If it's not a substantial win,
> I don't think we should accept the change --- the portability risks and
> testing/maintenance costs loom too large for me.
>
I was considering it because you can use a much larger amount of shared
memory without reconfiguring the kernel.
> BTW, how exactly is mmap a substitute for SysV shared memory? AFAICT
> it's only defined to map a disk file into your address space, not to
> allow a shared memory region to be set up that's independent of any
> disk file.
It allows no backing store on disk. It is the BSD solution to SysV
share memory. Here are all the BSDi flags:
MAP_ANON Map anonymous memory not associated with any specific file. The file descriptor used for
creatingMAP_ANON must be -1. The offset parameter is ignored.
MAP_FIXED Do not permit the system to select a different address than the one specified. If the
specifiedaddress cannot be used, mmap will fail. If MAP_FIXED is specified, addr must be a
multiple of the pagesize. Use of this option is discouraged.
MAP_PRIVATE Modifications are private.
MAP_SHARED Modifications are shared.
We would use MAP_ANON|MAP_SHARED I guess.
-- Bruce Momjian | http://candle.pha.pa.us pgman@candle.pha.pa.us | (610)
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