From mboxrd@z Thu Jan 1 00:00:00 1970 From: George Dunlap Subject: Re: [PATCHv2 0/3] Implement per-cpu reader-writer locks Date: Tue, 24 Nov 2015 18:30:53 +0000 Message-ID: <5654ACDD.5050107@citrix.com> References: <1448035423-24242-1-git-send-email-malcolm.crossley@citrix.com> <5654A98D.3050801@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta5.messagelabs.com ([195.245.231.135]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1a1IMl-0006PY-D2 for xen-devel@lists.xenproject.org; Tue, 24 Nov 2015 18:30:59 +0000 In-Reply-To: <5654A98D.3050801@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Malcolm Crossley , JBeulich@suse.com, ian.campbell@citrix.com, andrew.cooper3@citrix.com, Marcos.Matsunaga@oracle.com, keir@xen.org, konrad.wilk@oracle.com, george.dunlap@eu.citrix.com Cc: xen-devel@lists.xenproject.org, stefano.stabellini@citrix.com List-Id: xen-devel@lists.xenproject.org On 24/11/15 18:16, George Dunlap wrote: > On 20/11/15 16:03, Malcolm Crossley wrote: >> This patch series adds per-cpu reader-writer locks as a generic lock >> implementation and then converts the grant table and p2m rwlocks to >> use the percpu rwlocks, in order to improve multi-socket host performance. >> >> CPU profiling has revealed the rwlocks themselves suffer from severe cache >> line bouncing due to the cmpxchg operation used even when taking a read lock. >> Multiqueue paravirtualised I/O results in heavy contention of the grant table >> and p2m read locks of a specific domain and so I/O throughput is bottlenecked >> by the overhead of the cache line bouncing itself. >> >> Per-cpu read locks avoid lock cache line bouncing by using a per-cpu data >> area to record a CPU has taken the read lock. Correctness is enforced for the >> write lock by using a per lock barrier which forces the per-cpu read lock >> to revert to using a standard read lock. The write lock then polls all >> the percpu data area until active readers for the lock have exited. >> >> Removing the cache line bouncing on a multi-socket Haswell-EP system >> dramatically improves performance, with 16 vCPU network IO performance going >> from 15 gb/s to 64 gb/s! The host under test was fully utilising all 40 >> logical CPU's at 64 gb/s, so a bigger logical CPU host may see an even better >> IO improvement. > > Impressive -- thanks for doing this work. > > One question: Your description here sounds like you've tested with a > single large domain, but what happens with multiple domains? > > It looks like the "per-cpu-rwlock" is shared by *all* locks of a > particular type (e.g., all domains share the per-cpu p2m rwlock). > (Correct me if I'm wrong here.) Sorry, looking in more detail at the code, it seems I am wrong. The fast-path stores which "slow" lock has been grabbed in the per-cpu variable; so the writer only needs to wait for readers that have grabbed the particular lock it's interested in. So the scenarios I outline below shouldn't really be issues. The description of the algorithm in the changelog could do with a bit more detail. :-) -George