Re: [PATCH 1/1] Revert "iommu/iova: Retry from last rb tree node if iova search fails"

[John Garry]

On 01/03/2021 15:48, John Garry wrote:
> > While max32_alloc_size indirectly tracks the largest*contiguous* 
> > available space, one of the ideas from which it grew was to simply keep
> > count of the total number of free PFNs. If you're really spending
> > significant time determining that the tree is full, as opposed to just
> > taking longer to eventually succeed, then it might be relatively
> > innocuous to tack on that semi-redundant extra accounting as a
> > self-contained quick fix for that worst case.
> >
> > > Anyway, we see ~50% throughput regression, which is intolerable. As seen
> > > in [0], I put this down to the fact that we have so many IOVA requests
> > > which exceed the rcache size limit, which means many RB tree accesses
> > > for non-cacheble IOVAs, which are now slower.
>
> I will attempt to prove this by increasing RCACHE RANGE, such that all 
> IOVA sizes may be cached.

About this one, as expected, we restore performance by increasing the 
RCACHE RANGE.

Some figures:
Baseline v5.12-rc1

strict mode:
600K IOPs

Revert "iommu/iova: Retry from last rb tree node if iova search fails":
1215K

Increase IOVA RCACHE range 6 -> 10 (All IOVAs size requests now 
cacheable for this experiment):
1400K

Reduce LLDD max SGE count 124 -> 16:
1288K

non-strict mode
1650K

So ideally we can work towards something for which IOVAs of all size 
could be cached.

Cheers,
John