Re: Actually using the sg table/chain code

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2008-01-15 at 18:09 +0200, Boaz Harrosh wrote:
> On Tue, Jan 15 2008 at 17:52 +0200, James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> > I thought, now we had this new shiny code to increase the scatterlist
> > table size I'd try it out.  It turns out there's a pretty vast block
> > conspiracy that prevents us going over 128 entries in a scatterlist.
> > 
> > The first problems are in SCSI:  The host parameters sg_tablesize and
> > max_sectors are used to set the queue limits max_hw_segments and
> > max_sectors respectively (the former is the maximum number of entries
> > the HBA can tolerate in a scatterlist for each transaction, the latter
> > is a total transfer cap on the maxiumum number of 512 byte sectors).
> > The default settings, assuming the HBA doesn't vary them are
> > sg_tablesize at SG_ALL (255) and max_sectors at SCSI_DEFAULT_MAX_SECTORS
> > (1024).  A quick calculation shows the latter is actually 512k or 128
> > pages (at 4k pages), hence the persistent 128 entry limit.
> > 
> > However, raising max_sectors and sg_tablesize together still doesn't
> > help:  There's actually an insidious limit sitting in the block layer as
> > well.  This is what blk_queue_max_sectors says:
> > 
> > void blk_queue_max_sectors(struct request_queue *q, unsigned int
> > max_sectors)
> > {
> > 	if ((max_sectors << 9) < PAGE_CACHE_SIZE) {
> > 		max_sectors = 1 << (PAGE_CACHE_SHIFT - 9);
> > 		printk("%s: set to minimum %d\n", __FUNCTION__, max_sectors);
> > 	}
> > 
> > 	if (BLK_DEF_MAX_SECTORS > max_sectors)
> > 		q->max_hw_sectors = q->max_sectors = max_sectors;
> >  	else {
> > 		q->max_sectors = BLK_DEF_MAX_SECTORS;
> > 		q->max_hw_sectors = max_sectors;
> > 	}
> > }
> > 
> > So it imposes a maximum possible setting of BLK_DEF_MAX_SECTORS which is
> > defined in blkdev.h to .... 1024, thus also forcing the queue down to
> > 128 scatterlist entries.
> > 
> > Once I raised this limit as well, I was able to transfer over 128
> > scatterlist elements during benchmark test runs of normal I/O (actually
> > kernel compiles seem best, they hit 608 scatterlist entries).
> > 
> > So my question, is there any reason not to raise this limit to something
> > large (like 65536) or even eliminate it altogether?
> > 
> > James
> > 
> I have an old branch here where I've swiped through the scsi drivers just
> to remove the SG_ALL limit. Unfortunately some drivers mean laterally
> 255 when using SG_ALL. So I passed driver by driver and carfully inspected
> the code to change it to something driver specific if they really meant
> 255.
> 
> I have used sg_tablesize = ~0; to indicate, I don't care any will do,
> and some driver constant if there is a real limit. Though removing
> SG_ALL at the end.
> 
> Should I freshen up this branch and send it.

By all means; however, I think having the defined constant SG_ALL is
useful (even if it is eventually just set to ~0) it means I can support
any scatterlist size.  Having the drivers set sg_tablesize correctly
that can't support SG_ALL is pretty vital.

Thanks,

James