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From: Ralf.Brown@B.GP.CS.CMU.EDU
Newsgroups: comp.os.msdos.programmer
Subject: Re: Performance penalty from EMM386 ?!?!
Message-ID: <286a21ba@ralf>
Date: 27 Jun 91 17:34:50 GMT
Organization: Carnegie Mellon University School of Computer Science
Lines: 33
In-Reply-To: <13664@pt.cs.cmu.edu>
Originator: ralf@B.GP.CS.CMU.EDU

In article <13664@pt.cs.cmu.edu>, ralf+@cs.cmu.edu (Ralf Brown) wrote:
}In article <1991Jun26.120951.18540@cbfsb.att.com> feg@cbnewsb.cb.att.com (forrest.e.gehrke) writes:
}A slight slowdown is to be expected when running in virtual-86 mode
}[going between V86 and protected mode for interrupts]
}and finally returns to V86 mode for a second time to complete the
}program's interrupt call.  QEMM does all of that much faster than EMM386--
}Manifest reports that the timer interrupt latency is on average five
}times as high for EMM386 as for QEMM; comparing worst cases for the two
}yields a factor of *twelve*.  It can take over TWO MILLISECONDS for
}Manifest to get control once the clock interrupt is invoked, when running
}under EMM386 on my 386/33.

I wrote that from memory, and got a couple of the timings confused.
Below are the Manifest reports for QEMM v5.11 and EMM386.SYS, both under
plain DOS 5.0 and DOS 5.0 plus DESQview 2.31.  Note that DV adds some
overhead on the timer interrupt, but EMM386 is *so*bad* that DV actually
*improves* the average and worst-case performance!

                              (times in microseconds)
         EMS Function        Minimum  Maximum  Average
QEMM, DOS 5.0
    Timer Interrupt Latency       6       53       25
EMM386.SYS, DOS 5.0
    Timer Interrupt Latency       8     2036      316
QEMM, DOS5+DV
    Timer Interrupt Latency       5      156       59
EMM386.SYS, DOS5+DV
    Timer Interrupt Latency      46     1030      245
-- 
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