linux-kernel - Re: [take21 0/4] kevent: Generic event handling mechanism.

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Message-Id: <20061107.125241.39157521.davem@davemloft.net>
Date:	Tue, 07 Nov 2006 12:52:41 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	akpm@...l.org
Cc:	jeff@...zik.org, johnpol@....mipt.ru, drepper@...hat.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	torvalds@...l.org
Subject: Re: [take21 0/4] kevent: Generic event handling mechanism.

From: Andrew Morton <akpm@...l.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.
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