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Date: Thu, 20 Apr 2017 10:13:26 -0700 From: Davidlohr Bueso <dave@...olabs.net> To: Peter Zijlstra <peterz@...radead.org> Cc: mingo@...nel.org, akpm@...ux-foundation.org, jack@...e.cz, kirill.shutemov@...ux.intel.com, ldufour@...ux.vnet.ibm.com, mhocko@...e.com, mgorman@...hsingularity.net, linux-kernel@...r.kernel.org Subject: Re: [PATCH v2 -tip 0/6] locking: Introduce range reader/writer lock On Wed, 19 Apr 2017, Peter Zijlstra wrote: > - explain why the loss of lock stealing makes sense. IIRC walken added > that specifically to address mmap_sem performance issues. That's right, and the same applies to the writer spinning stuff; which can makes a huge difference - more so than plain stealing. But as I've mentioned, range locks can improve parallelism, which is/should be much more welcomed than optimizations to the primitive. So yeah, we loose in comparing a full range to rwsem (not to mention the xadd stuff). I have thought of some heuristics for avoiding sleeping under certain constraints, which could mitigate the spinning step we loose, but I fear it will never be exactly as fast as rwsems -- just consider we always take the tree->lock. Thanks, Davidlohr
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