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From: David Miller <davem@davemloft.net>
Subject: Re: [take21 0/4] kevent: Generic event handling mechanism.
Date: Tue, 07 Nov 2006 12:52:41 -0800 (PST)
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From: Andrew Morton <akpm@osdl.org>
Date: Tue, 7 Nov 2006 11:34:00 -0800

> What Evgeniy means here is that copy_to_user() is slower than memcpy() (on
> his machine, with his kernel config, at least).
> 
> Which is kinda weird and unexpected and is something which we should
> investigate independently from this project.  (Rather than simply going
> and bypassing it!)

It's straightforward to me. :-)

If the kerne memcpy()'s, it uses those nice 4MB PTE mappings to
the kernel pages.  With copy_to_user() you run through tiny
4K or 8K PTE mappings which thrash the TLB.

The TLB is therefore able to hold more of the accessed state at
a time if you touch the pages on the kernel side.