From: Ralf Baechle <ralf@oss.sgi.com>
To: Justin Carlson <carlson@sibyte.com>
Cc: linux-mips@oss.sgi.com
Subject: Re: _clear_page semantics
Date: Wed, 10 Jan 2001 03:03:26 -0200 [thread overview]
Message-ID: <20010110030326.B8489@bacchus.dhis.org> (raw)
In-Reply-To: <01010917590106.07691@plugh.sibyte.com>; from carlson@sibyte.com on Tue, Jan 09, 2001 at 05:48:11PM -0800
On Tue, Jan 09, 2001 at 05:48:11PM -0800, Justin Carlson wrote:
> Looking at the existing clear_page implementations for r4xx0, rm7k, and mips32
> in the mips/ tree, I see everyone issuing cache op 0xd for the address range of
> the page being cleared.
>
> I'm wondering what the purpose is of these cache flushes...given a physically
> tagged dcache, my understanding of the semantics of clear_page are that it just
> zeros the page, in which case the cache ops are pointless overhead.
>
> Especially in the mips32 case, which uses cache op 0xd, which is undefined
> implementation dependent according to my mips32 spec.
The idea is to avoid unnecessary memory reads - all the read data would
be overwritten anyway. The last time I benchmarked this routine on some
machine it made a difference of about 4000 vs. 2500 c0_count cycles. I
think that was on a R4600 RM200.
Ralf
next prev parent reply other threads:[~2001-01-10 5:14 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-01-10 1:48 _clear_page semantics Justin Carlson
2001-01-10 5:03 ` Ralf Baechle [this message]
2001-01-10 8:08 ` Carsten Langgaard
2001-01-10 15:34 ` Ralf Baechle
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