[8194] in linux-scsi channel archive
Re: scatter gather or not to scatter gather
daemon@ATHENA.MIT.EDU (Matthew Dharm)
Wed Feb 23 19:45:01 2000
Date: Wed, 23 Feb 2000 12:19:20 -0800 (PST)
From: Matthew Dharm <mdharm-scsi@one-eyed-alien.net>
To: Douglas Gilbert <dgilbert@interlog.com>
Cc: The Linux SCSI list <linux-scsi@vger.rutgers.edu>
In-Reply-To: <38B3E1A8.F6F6CC36@interlog.com>
Message-ID: <Pine.LNX.4.10.10002231216390.11240-100000@ziggy.one-eyed-alien.net>
MIME-Version: 1.0
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On Wed, 23 Feb 2000, Douglas Gilbert wrote:
> Matthew Dharm wrote:
> >
> > Well, I spoke too soon, apparently.
> >
> > After doing some tests with SG_NONE, I discovered that instead of issuing
> > one large transfer, the SCSI layers above me issue lots of little (1K)
> > transfers. Effectively, the scatter-gather behavior is being emulated,
> > but with much more overhead involved.
> >
> > Is there any way to tell the higher scsi layers "I would like big
> > transfers, with larger chunks of memory"? The memory seems available...
> > often two or three successive transfers come in with adjacent memory
> > buffers.
>
> Matt,
> I'm not aware of a way for a HBA to tell the higher layers anything
> about desired chunk sizes. In the case of sg, the users of sg (SANE,
> cdparanoia, etc) like to control that parameter ... Then there are
> restrictions of the kernel and the HBA (e.g. ISA) which make this area
> reasonably complicated.
Hrm... is there any desire to add the ability for the HBA to convey this
information? If I'm the only one that has a desire for this, then let's
not bother. But would anyone else's drivers work better if they could
convey some sort of preferred chunk size?
> BTW Sg uses a 32KB chunk size so to support a transfer size of 128KB
> that SANE and cdparanoia like requires the HBA to support a scatter
> gather list of at least 4 elements. Will USB scanners be using your
> driver? If so, that is one reason to support scatter gather.
No. USB Scanners will use the USB Scanner driver. I don't know much
about the internals of this driver, but it is being actively maintained.
Matt Dharm
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