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<TITLE>sane-devel: Re: scsi command queuing</TITLE>
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<META NAME="Author" CONTENT="Oliver Rauch (oliver.rauch@Wolfsburg.DE)">
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<META NAME="Subject" CONTENT="Re: scsi command queuing">
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<H1>Re: scsi command queuing</H1>
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<!-- received="Sun Jul 2 14:31:21 2000" -->
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<!-- isoreceived="20000702213121" -->
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<!-- sent="Sun, 02 Jul 2000 23:47:57 +0200" -->
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<!-- name="Oliver Rauch" -->
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<!-- email="oliver.rauch@Wolfsburg.DE" -->
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<!-- subject="Re: scsi command queuing" -->
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<!-- id="395FB88D.545E215B@wolfsburg.de" -->
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<!-- inreplyto="395F7773.68AB4289@satzbau-gmbh.de" -->
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<STRONG>From:</STRONG> Oliver Rauch (<A HREF="mailto:oliver.rauch@Wolfsburg.DE?Subject=Re:%20scsi%20command%20queuing&In-Reply-To=<395FB88D.545E215B@wolfsburg.de>"><EM>oliver.rauch@Wolfsburg.DE</EM></A>)<BR>
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<STRONG>Date:</STRONG> Sun Jul 02 2000 - 14:47:57 PDT
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<P>
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<LI><STRONG>Next message:</STRONG> <A HREF="0008.html">Didier Galland: "RE: can't recognize UMAX-VistaS6 on AHA1520b"</A>
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<LI><STRONG>Previous message:</STRONG> <A HREF="0006.html">Oliver Rauch: "Re: can't recognize UMAX-VistaS6 on AHA1520b"</A>
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<LI><STRONG>In reply to:</STRONG> <A HREF="0004.html">abel deuring: "Re: scsi command queuing"</A>
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<LI><STRONG>Next in thread:</STRONG> <A HREF="0015.html">abel deuring: "Re: scsi command queuing"</A>
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<HR NOSHADE><P>
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<P>
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abel deuring wrote:
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<BR>
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<P><EM>>
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</EM><BR>
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<EM>>
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</EM><BR>
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<EM>> I hope it too :) But, considering the discussion in this thread, it
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</EM><BR>
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<EM>> seems that very short delays between two SCSI commands or huge buffers
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</EM><BR>
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<EM>> are for many scanners the only ways to avoid scan head stops.
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</EM><BR>
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<EM>>
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</EM><BR>
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<P>The problem is that with large buffers and driver (not adapter) queue commands
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<BR>
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the scanhead does stop when the size of the scanner internal buffer is transmitted.
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<BR>
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So it looks like a response time problem of the low level scsi driver.
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<BR>
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<P><P><EM>>
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</EM><BR>
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<EM>> DBG(1, "issue: EAGAIN - cannot queue SCSI command. "
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</EM><BR>
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<EM>> "Trying again later.\n");
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</EM><BR>
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<EM>>
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</EM><BR>
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<P>No such mesages.
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<BR>
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<P><EM>>
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</EM><BR>
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<EM>>
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</EM><BR>
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<EM>> I think that at least some choice of the buffer size should be left to
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</EM><BR>
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<EM>> the user: You can never know what kind of programs and how many of them
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</EM><BR>
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<EM>> somebody might run at the same time on her/his Linux box. and which
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</EM><BR>
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<EM>> priority the scanner usage has: If the backend fights too hard for
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</EM><BR>
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<EM>> memory, this might cause a serious slowdown of other, perhaps more
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</EM><BR>
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<EM>> important, programs, because their memory content is written to the swap
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</EM><BR>
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<EM>> disk.
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</EM><BR>
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<P>Yes.
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<BR>
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<P><EM>>
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</EM><BR>
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<EM>>
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</EM><BR>
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<EM>> But "memory policies" aside, you are right, we must find a way to avoid
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</EM><BR>
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<EM>> scan head stops. But I don't see many options without messing in the
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</EM><BR>
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<EM>> Linux kernel. At least, I don't see any way to speed things more up than
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</EM><BR>
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<EM>> to use command queueing and more or less big buffers without asking for
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</EM><BR>
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<EM>> faster kernel responses or new ioctls, like the idea to implement
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</EM><BR>
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<EM>> something similar to piping for the handling of really large SCSI read
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</EM><BR>
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<EM>> commands. (And the latter can be really dangerous: If we have a scanner
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</EM><BR>
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<EM>> that is unable to do a SCSI disconnect, and a disk, to which the image
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</EM><BR>
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<EM>> data should be written, connected to the same SCSI bus, there no way to
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</EM><BR>
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<EM>> get access to the disk while the scan is running.)
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</EM><BR>
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<EM>>
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</EM><BR>
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<P>I already played a bit with some kernel options/changes.
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<BR>
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So I tested forced sync and forced command queueing
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<BR>
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(the scanner says both are not possible but some devices
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<BR>
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that the can not do this although the really can), but this
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<BR>
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did not change anything as far as I tested yet.
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<BR>
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<P>So the next thing will be to play around with the sg or the low level scsi driver.
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<BR>
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<P>Bye
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<BR>
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Oliver
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<BR>
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<P><P><PRE>
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--
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Homepage: <A HREF="http://www.wolfsburg.de/~rauch">http://www.wolfsburg.de/~rauch</A>
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sane-umax: <A HREF="http://www.wolfsburg.de/~rauch/sane/sane-umax.html">http://www.wolfsburg.de/~rauch/sane/sane-umax.html</A>
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xsane: <A HREF="http://www.wolfsburg.de/~rauch/sane/sane-xsane.html">http://www.wolfsburg.de/~rauch/sane/sane-xsane.html</A>
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E-Mail: mailto:<A HREF="mailto:Oliver.Rauch@Wolfsburg.DE?Subject=Re:%20scsi%20command%20queuing&In-Reply-To=<395FB88D.545E215B@wolfsburg.de>">Oliver.Rauch@Wolfsburg.DE</A>
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<P><P><P>--
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Source code, list archive, and docs: <A HREF="http://www.mostang.com/sane/">http://www.mostang.com/sane/</A>
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To unsubscribe: echo unsubscribe sane-devel | mail <A HREF="mailto:majordomo@mostang.com?Subject=Re:%20scsi%20command%20queuing&In-Reply-To=<395FB88D.545E215B@wolfsburg.de>">majordomo@mostang.com</A>
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</PRE>
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<LI><STRONG>Next message:</STRONG> <A HREF="0008.html">Didier Galland: "RE: can't recognize UMAX-VistaS6 on AHA1520b"</A>
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<LI><STRONG>In reply to:</STRONG> <A HREF="0004.html">abel deuring: "Re: scsi command queuing"</A>
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<EM>
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This archive was generated by <A HREF="http://www.hypermail.org/">hypermail 2b29</A>
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: <EM>Sun Jul 02 2000 - 14:32:17 PDT</EM>
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