Linus Torvalds on FreeBSD's Use of Copy-on-write

[Alexander Kabaev]

On Sun, 23 Apr 2006 23:55:23 -0700
Julian Elischer <julian at elischer.org> wrote:

> John-Mark Gurney wrote:
> 
> >Kirk McKusick wrote this message on Sun, Apr 23, 2006 at 23:33 -0700:
> >  
> >
> >>Linus explained that while this may look good on specific
> >>benchmarks, it actually introduces extra overhead, "the thing is,
> >>the cost of
> >>    
> >>
> >
> >Has he benchmarked this to prove his point?  And has he done it over
> >realworld work loads, like Apache or another "standard" program
> >instead of a microbenchmark designed especially to make COW look bad?
> >  
> >
> 
> Well no-one has even confirmed that freeBSD does all this
> "page flipping" etc.  I doubt that Linus has looked inside the BSD
> kernels. He's probably just repeating what he's been told, and that's
> so accurate, right?
> 
> I know that freeBSD developers have over the last few years also 
> acknowledged that
> the speed  of modern CPUs vs. memeory speeds makes it often less
> efficient to do certain optimisations than it used to be.
> 
> 
> 
> 
> >As w/ all theories, w/o numbers, they are only theories till backed
> >up w/ benchmarks.
> >
> >This isn't suppose to defend COW, but it is designed to ensure that
> >people don't stop exploring just because someone says something...
> >
> >  
> >
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The original zero-copy code was used to stream network data directly to
an onboard SDRAM memory on an extension PCI card. Linus' notes about
relative cost of memory-to-memory copies vs. TLB shootdowns and possible
page access traps had little relevance there.

--
Alexander Kabaev
-- 
Alexander Kabaev
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