On 08/27/2014 02:46 PM, Andrew Morton wrote:

I assume (because I wasn't told!) that there are two objectives here:

1) reduce memory consumption by not maintaining pagecache and
2) reduce CPU cost by avoiding the double-copies.

These things are pretty easily quantified.  And really they must be
quantified as part of the developer testing, because if you find
they've worsened then holy cow, what went wrong.

There are two more huge ones:

3) Writes via mmap are immediately durable (or at least they're durable
after a *very* lightweight flush).

4) No page faults ever once a page is writable (I hope -- I'm not sure
whether this series actually achieves that goal).

A note on #3: there is ongoing work to enable write-through memory for
things like this.  Once that's done, then writes via mmap might actually
be synchronously durable, depending on chipset details.

--Andy

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