# Re: scsi command queuing

From: Henning Meier-Geinitz (hmg-ml@gmx.de)
Date: Wed Jun 28 2000 - 11:24:00 PDT

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  Hi,

  On Wed, Jun 28, 2000 at 05:26:23PM +0200, Oliver Rauch wrote:
  > has someone experience with a sane backend and scsi command queueing?

  I have tried this some weeks ago without much success. To be exact: it is
  possible to send more than one scsi_req_enter before scsi_req_wait and there
  is no problem with this. But it isn't faster than waiting for each request
  and then sending the next. I haven't looked deeply into the code (and I
  don't understand much of SCSI) but the following lines looked suspicous:
  (in scsi_rq_wait())
  /* Now issue next command asap, if any. We can't do this
  earlier since the Linux kernel has space for just one big
  buffer. */
  issue (req->next);

  So if I understand this correctly the scsi_req_enter only shedules the
  request and it will be sent to the driver whenever any pending request is
  finished. Maybe the "Linux can only use one big buffer" is history with the
  newer sg drivers?

  > Unfortunetly it does not speed up scanning large images. It really looks like
  > the comunication via the scsi bus is not fast enough.

  Same here. I looked at the scanning times when the backend does nothing but
  getting data from the scanner and ignoring it. There was no big change in
  scanning time (about 5 %). With the original SCSI adapter the Mustek
  scanners are about twice as slow as with Windows despite large (4 MB) SCSI
  buffers and tweaking the Linux SCSI driver.

  Bye,
  Henning

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