Re: [patch] SMP alternatives

[linux@horizon.com]

> I suspect that with MAP_SHARED + PROT_WRITE being pretty uncommon anyway, 
> we can probably find trivial patterns in the kernel. Like only one process 
> holding that file open - which is what you get with things that use mmap() 
> to write a new file (I think "ld" used to have a config option to write 
> files that way, for example).

Just a bit of practical experience: I use mmap() to write data a LOT,
because msync(MS_ASYNC) is the most portable way to do an async write.

There are two applications.  First, helping the OS not fill up with
dirty pages.  It's basically a way of saying "this page is not going to
be dirtied again for a long time".

Secondly, to reduce the latency of synchronous writes.  If I need to
log operations durably, it helps to

1) fill the log pages, using MS_ASYNC as soon as the page is full
2) when committing a batch, use MS_SYNC to force data to disk
3) report batch successfully committed to stable storage

The aio_ routines are less widely supported some implementations have
very high overhead.  They would allow me to keep working while a commit
is in progress, but the above is simple and reduces the burstiness of
I/O considerably.
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