On Tue, Jan 15 2008 at 17:52 +0200, James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> 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.
Boaz
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