On Thu, 20 Apr 2006, Piet Delaney wrote:
>
> What about marking the pages Read-Only while it's being used by the
> kernel
NO!
That's a huge mistake, and anybody that does it that way (FreeBSD) is
totally incompetent.
Once you play games with page tables, you are generally better off copying
the data. The cost of doing page table updates and the associated TLB
invalidates is simply not worth it, both from a performance standpoing and
a complexity standpoint.
Basically, if you want the highest possible performance, you do not want
to do TLB invalidates. And if you _don't_ want the highest possible
performance, you should just use regular write(), which is actually good
enough for most uses, and is portable and easy.
The thing is, the cost of marking things COW is not just the cost of the
initial page table invalidate: it's also the cost of the fault eventually
when you _do_ write to the page, even if at that point you decide that the
page is no longer shared, and the fault can just mark the page writable
again.
That cost is _bigger_ than the cost of just copying the page in the first
place.
The COW approach does generate some really nice benchmark numbers, because
the way you benchmark this thing is that you never actually write to the
user page in the first place, so you end up having a nice benchmark loop
that has to do the TLB invalidate just the _first_ time, and never has to
do any work ever again later on.
But you do have to realize that that is _purely_ a benchmark load. It has
absolutely _zero_ relevance to any real life. Zero. Nada. None. In real
life, COW-faulting overhead is expensive. In real life, TLB invalidates
(with a threaded program, and all users of this had better be threaded, or
they are leaving more performance on the floor) are expensive.
I claim that Mach people (and apparently FreeBSD) are incompetent idiots.
Playing games with VM is bad. memory copies are _also_ bad, but quite
frankly, memory copies often have _less_ downside than VM games, and
bigger caches will only continue to drive that point home.
Linus
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