[PATCHES] A patch for xlog.c

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> 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 creating MAP_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 specified address 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.

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