Date: Wed, 19 Jul 2017 16:00:55 -0700
From: Davidlohr Bueso <dave@stgolabs.net>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: mingo@kernel.org, peterz@infradead.org, torvalds@linux-foundation.org,
        jack@suse.cz, kirill.shutemov@linux.intel.com,
        ldufour@linux.vnet.ibm.com, mhocko@suse.com,
        mgorman@techsingularity.net, linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next v3 0/9] rbtree: Cache leftmost node internally
Message-ID: <20170719230055.GE14395@linux-80c1.suse>
References: <20170629171553.2146-1-dave@stgolabs.net>
 <20170719155407.6002707adc68e8de833ffe49@linux-foundation.org>
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On Wed, 19 Jul 2017, Andrew Morton wrote:

>On Thu, 29 Jun 2017 10:15:44 -0700 Davidlohr Bueso <dave@stgolabs.net> wrote:
>
>> Changes from v2 (https://lkml.org/lkml/2017/6/8/857):
>> - Fixed 0day reported crash for drm_mm selftest program. We were
>> not correctly using the cached version of rbtree with the allocated
>> nodes.
>> - Added cfq patch to use internal rbtree caching.
>> - Added Christian's and Jan's reviews.
>>
>> Changes from v1 (https://marc.info/?l=linux-kernel&m=149611025616685):
>> - No longer rfc.
>> - Removed bogus semimcolon in rb_first_cached()
>> - Updated missing interval tree user drivers/infiniband/hw/hfi1/
>> - Removed redundant @cached arg in when erasing a node.
>> - Added more patches that make use of rb_first_cached(), which I
>>   thought might be worth it: procfs and epoll.
>> - Cc more people for patch 5, which touches drivers such as infiniband
>> and gpu. The rest of the changes are pretty covered with the current
>> Cc'ed maintainers and mm folks.
>>
>> Hi,
>>
>> Here's a proposal for extending rbtrees to internally cache the leftmost
>> node such that we can have fast overlap check optimization for all interval
>> tree users[1]. The benefits of this series are that:
>>
>> (i)   Unify users that do internal leftmost node caching.
>
>That's nice.  Except the series adds more lines than it removes.
>
>> (ii)  Optimize all interval tree users.
>
>Was any attempt made to quantify the benefit?

Yes, but ultimately it will depend a lot on the workload and size of the tree.
For bare numbers, on a Xeon E5-2450 @ 2.10GHz the cost of a rb_first() was
~60 cycles for 100 nodes, and ~75 cycles with 1000 nodes. fwiw.

Thanks,
Davidlohr