[8174] in linux-scsi channel archive
Re: scatter gather or not to scatter gather
daemon@ATHENA.MIT.EDU (Matthew Dharm)
Wed Feb 23 03:41:41 2000
Date: Tue, 22 Feb 2000 22:47:11 -0800 (PST)
From: Matthew Dharm <mdharm-scsi@one-eyed-alien.net>
To: The Linux SCSI list <linux-scsi@vger.rutgers.edu>
In-Reply-To: <Pine.LNX.4.10.10002221759010.6488-100000@ziggy.one-eyed-alien.net>
Message-ID: <Pine.LNX.4.10.10002222244460.15525-100000@ziggy.one-eyed-alien.net>
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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 Dharm
On Tue, 22 Feb 2000, Matthew Dharm wrote:
> To scatter-gather, or not to scatter-gather; that is the question.
>
> Basically, I'm writting a virtual host driver. I can support scatter
> gather, or I can opt not to. If I don't use scatter gather, there will be
> an (as yet) unquantified performance gain on the USB bus that this virtual
> host uses. However, what are the negatives in terms of the SCSI
> layering?
>
> >From my logs, it seems that the SCSI layer is sending me long
> scatter-gather chains, where each block is 1K in size, and they tend to be
> all or mostly adjacent in RAM.
>
> The question boils down to this: Why would it be bad for me to replace the
> SG_ALL in the host template with SG_NONE?
>
> Matt Dharm
>
>
--
Matthew Dharm Home: mdharm@one-eyed-alien.net
Engineer, Qualcomm, Inc. Work: mdharm@qualcomm.com
What, are you one of those Microsoft-bashing Linux freaks?
-- Customer to Greg
User Friendly, 2/10/1999
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