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Subject: Re: Reading from /dev/fb0 very slow?
To: pavel@suse.cz (Pavel Machek)
Date: Sat, 2 Jun 2001 23:03:24 +0100 (BST)
Cc: linux-kernel@vger.kernel.org (kernel list)
In-Reply-To: <20010602105249.A979@bug.ucw.cz> from "Pavel Machek" at Jun 02, 2001 10:52:49 AM
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From: Alan Cox <alan@lxorguk.ukuu.org.uk>
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> I did some benchmarks, and my framebuffer is *way* faster when writing
> than when reading:

That is quite normal.

> That is 6 times slower! This is also very visible in X, where moving
> regions is expensive, while just drawing regions is fast. For example
> gnome-terminal is *way* faster *with* transparent background option.
> 
> Any idea why such assymetry? [This is toshiba 4030cdt with vesafb and
> 2.4.5]

Writes to a PCI device can be queued or posted. Reads from a PCI device for
obvious reasons have to stall the CPU until the data returns.