From: Andrew Morton <[email protected]>
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.
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