Re: Actually using the sg table/chain code

[Boaz Harrosh <bharrosh@panasas.com>]

On Wed, Jan 16 2008 at 17:09 +0200, James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> On Wed, 2008-01-16 at 16:01 +0200, Boaz Harrosh wrote:
>> On Tue, Jan 15 2008 at 19:35 +0200, Boaz Harrosh <bharrosh@panasas.com> wrote:
>>> On Tue, Jan 15 2008 at 18:49 +0200, James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
>>>> 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
>>> OK will do.
>>>
>>> I have found the old branch and am looking. I agree with you about the 
>>> SG_ALL. I will fix it to have a patch per changed driver, with out changing
>>> SG_ALL, and then final patch to just change SG_ALL.
>>>
>>> Boaz
>>
>> James hi
>> reinspecting the code, what should I do with drivers that do not support chaining
>> do to SW that still do sglist++?
>>
>> should I set their sg_tablesize to SG_MAX_SINGLE_ALLOC, or hard code to 128, and put
>> a FIXME: in the submit message?
>>
>> or should we fix them first and serialize this effort on top of those fixes.
>> (also in light of the other email where you removed the chaining flag)
> 
> How many of them are left?
> 
> The correct value is clearly SCSI_MAX_SG_SEGMENTS which fortunately
> "[PATCH] remove use_sg_chaining" moved into a shared header.  Worst
> case, just use that and add a fixme comment giving the real value (if
> there is one).
> 
> James
> 
> 

I have 9 up to now and 10 more drivers to check. All but one are
SW, one by one SCp.buffer++, so once it's fixed they should be able
to go back to SG_ALL. But for now I will set them to SCSI_MAX_SG_SEGMENTS
as you requested. I have not checked drivers that did not use SG_ALL
but I trust these are usually smaller.

Boaz