Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!m2c!ulowell!masscomp!black
From: black@masscomp.UUCP (Sam Black)
Newsgroups: comp.windows.x
Subject: Re: Graphics Context in X11
Keywords: caching, cost
Message-ID: <2819@masscomp.UUCP>
Date: 25 Jan 88 15:15:33 GMT
References: <5163X@utah-cs.UUCP>
Reply-To: black@masscomp.UUCP (Sam Black)
Organization: MASSCOMP - Westford, Ma
Lines: 34

> The X11 Xlib document warns against using too many graphics contexts and
> switching between them, as opposed to re-setting the attributes of a few.
> I would like to know:
>
> 1. Does anybody actually cache GC data in the hardware, and if so, what sorts
> of data and how much is typically cached?
>
> 2. What is the range in performance variation between using, say, 30 GCs whose
> attributes are never changed, vs one GC with an average of two attribute
> settings per graphics call?  I'm especially interested in differences between
> one server and another.

Our server implementation works just the opposite way:  it is MUCH faster
to switch between GCs than to change values in GCs, assuming you do
not switch windows (and thus recalculate clipping).  There are several
reasons:

	1) ValidateGC is avoided
	2) ChangeGC is avoided
	3) we cache GCs in the hardware (up to several hundred)

We haven't come up with actual timing differences, but it can be noticeable.

		- sam black

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