Re: mmap for zeroing WAL log

On Sat, 24 Feb 2001, Tom Lane wrote:

> >> I am confused why mmap() is better than writing to a real file.
>
> > It isn't, except that it allows to initialise the logfile in
> > one syscall, without first allocating and zeroing (and hence
> > dirtying) 16Mb of memory.
>
> Uh, the existing code does not zero 16Mb of memory... it zeroes
> 8K and then writes that block repeatedly.

See the "one syscall" bit above.

> It's possible that the overhead of a syscall for each 8K block is
> significant,

I had assumed that the overhead would come from synchronous
metadata incurring writes of at least the inode, block bitmap
and probably an indirect block for each syscall.

> but on the other hand writing a block at a time is a heavily used and
> heavily optimized path in all Unixen. It's at least as plausible that
> the mmap-as-source-of-zeroes path will be slower!

Results:

On Linux/ext2, it appears good for a gain of 3-5% for log
creations (via a fairly minimal test program).

On FreeBSD 4.1-RELEASE/ffs (with all of sync/async/softupdates)
it is a couple of percent worse in elapsed time, but consumes
around a third more system CPU time (12sec vs 9sec on one test
system).

I am awaiting numbers from reiserfs but, for now, it looks like
I am far from vindicated.

Matthew.