Re: CVS commit: [tls-maxphys] src/sys/dev

[Thor Lancelot Simon <tls%panix.com@localhost>]

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
> value)

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.

Thor