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Re: CVS commit: [tls-maxphys] src/sys/dev
On Wed, Oct 10, 2012 at 11:34:48AM +0200, Manuel Bouyer wrote:
> I'm not sure what the best way to handle this would be.
> If we assume that maxphys is a power of 2, we could use a maxphys-derived
> mask here. Otherwise, maybe we should compute and cache the largest power-of-2
> value below maxphys in v_mount, as is done in vfs_vnops.c:vn_ra_allocctx()
> (actually, this would remove ra_iochunk as we could use the mount point's
I originally did that. There were some negative performance consequences.
With two of the SCSI (RAID, actually) controllers I have on hand, the DMA
descriptor format effectively imposes a restriction of 192K on transfer
size. But these are exactly the sort of devices that really like to see
large writes. The next lower power of two is 128k...
I think a lot of the clever math with shifts and masks should perhaps go
away. Any efficiency improvements it yields are, I think, swamped by the
5,000 function calls we make to do a single I/O.
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