Re: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq_segments about VMERGE

[Mikulas Patocka <mpatocka@redhat.com>]

>>> On sparc64 it is broken anyway with or without your patch.
>>
>> Yeah, we need to modify SPARC64 IOMMU code (I'm not sure that it's
>> worth). Right now, the best fix is setting BIO_VMERGE_BOUNDARY to 0.
>>
>>
>>> And alpha alone doesn't justify substantial code bloat in generic block
>>> layer. So I propose this patch to drop it at all.
>>
>> Jens, what do you think about removing VMERGE code?
>
> Actually, it's code I did.
>
> There are plusses and minusses to all of this.  The original vmerge code
> was done for sparc ... mainly because the benefits of virtual merging
> can offset the cost of having to use the iommu.  However, most
> architectures didn't use it.  When I fixed it up to work for parisc (and
> introduced the parameters) we were trying to demonstrate that using it
> was feasible.
>
> The idea behind vmerging is that assembling and programming sg lists is
> expensive, so you want to do it once.  Either in the iommu or in the
> driver sg list, but not in both.  There is evidence that it saves around
> 7% or so on drivers.  However, for architectures that can do it, better
> savings are made simply by lifting the iommu out of the I/O path (so
> called bypass mode).

The problem is with vmerge accounting in block layer (that is what I'm 
proposing to remove), not with vmerge itself.

Vmerge accounting has advantages only if you have device with small amount 
of sg slots --- it allows the block layer to create request that has 
higher number of segments then the device.

If you have device with for example 1024 slots, the virtual merge 
accounting has no effect, because the any request will fit into that size. 
Even without virtual merge accounting, the virtual merging will happen, so 
there will be no performance penalty for the controller --- the controller 
will be programmed with exactly the same number of segments as if virtual 
merge accounting was present. (there could be even slight positive 
performance effect if you remove accounting, because you burn less CPU 
cycles per request)

If you have device will small number of sg slots (16 or so), vmerge 
accounting can improve performance by creating requests with more than 16 
segments --- the question is: is there any such device? And is the device 
performance-sensitive? (i.e. isn't it such an old hardware where no one 
cares about performance anyway?)

> I suspect with IOMMUs coming back (and being unable to be bypassed) with
> virtualisation, virtual merging might once more become a significant
> value.

I suppose that no one would manufacture new SCSI card with 16 or 32 sg 
slots these days, so the accounting of hardware segments has no effect on 
modern hardware.

Mikulas

> James
>
>