[Intel-gfx] [PATCH 20/41] drm/i915: Replace priolist rbtree with a skiplist

[Chris Wilson]

Quoting Matthew Brost (2021-01-28 22:56:04)
> On Mon, Jan 25, 2021 at 02:01:15PM +0000, Chris Wilson wrote:
> > Replace the priolist rbtree with a skiplist. The crucial difference is
> > that walking and removing the first element of a skiplist is O(1), but
> > O(lgN) for an rbtree, as we need to rebalance on remove. This is a
> > hindrance for submission latency as it occurs between picking a request
> > for the priolist and submitting it to hardware, as well effectively
> > trippling the number of O(lgN) operations required under the irqoff lock.
> > This is critical to reducing the latency jitter with multiple clients.
> > 
> > The downsides to skiplists are that lookup/insertion is only
> > probablistically O(lgN) and there is a significant memory penalty to
> > as each skip node is larger than the rbtree equivalent. Furthermore, we
> > don't use dynamic arrays for the skiplist, so the allocation is fixed,
> > and imposes an upper bound on the scalability wrt to the number of
> > inflight requests.
> > 
> 
> This is a fun data structure but IMO might be overkill to maintain this
> code in the i915. The UMDs have effectively agreed to use only 3 levels,
> is O(lgN) where N == 3 really a big deal? With GuC submission we will
> statically map all user levels into 3 buckets. If we are doing that, do
> we even need a complex data structure? i.e. Could use just use can
> array of linked lists?

Because we need to scale the bst to handle a unqiue key per request with
thousands of requests [this is not only about priorities]. And as you
will see from the results, even with just a single priority in the system
(so one entry in either the skiplist or rbtree), the skiplist is beating 
the rbtree as measured by the lock hold time around insert/dequeue of
requests. That surprised me.
-Chris