[dpdk-dev] [PATCH] lib: change rte_ring dequeue to guarantee ordering before tail update

[Jerin Jacob]

On Sat, Jul 23, 2016 at 12:32:01PM +0000, Ananyev, Konstantin wrote:
> 
> 
> > -----Original Message-----
> > From: Jerin Jacob [mailto:jerin.jacob at caviumnetworks.com]
> > Sent: Saturday, July 23, 2016 12:49 PM
> > To: Ananyev, Konstantin <konstantin.ananyev at intel.com>
> > Cc: Thomas Monjalon <thomas.monjalon at 6wind.com>; Juhamatti Kuusisaari <juhamatti.kuusisaari at coriant.com>; dev at dpdk.org
> > Subject: Re: [dpdk-dev] [PATCH] lib: change rte_ring dequeue to guarantee ordering before tail update
> > 
> > On Sat, Jul 23, 2016 at 11:15:27AM +0000, Ananyev, Konstantin wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: Jerin Jacob [mailto:jerin.jacob at caviumnetworks.com]
> > > > Sent: Saturday, July 23, 2016 11:39 AM
> > > > To: Ananyev, Konstantin <konstantin.ananyev at intel.com>
> > > > Cc: Thomas Monjalon <thomas.monjalon at 6wind.com>; Juhamatti
> > > > Kuusisaari <juhamatti.kuusisaari at coriant.com>; dev at dpdk.org
> > > > Subject: Re: [dpdk-dev] [PATCH] lib: change rte_ring dequeue to
> > > > guarantee ordering before tail update
> > > >
> > > > On Sat, Jul 23, 2016 at 10:14:51AM +0000, Ananyev, Konstantin wrote:
> > > > > Hi lads,
> > > > >
> > > > > > On Sat, Jul 23, 2016 at 11:02:33AM +0200, Thomas Monjalon wrote:
> > > > > > > 2016-07-23 8:05 GMT+02:00 Jerin Jacob <jerin.jacob at caviumnetworks.com>:
> > > > > > > > On Thu, Jul 21, 2016 at 11:26:50PM +0200, Thomas Monjalon wrote:
> > > > > > > >> > > Consumer queue dequeuing must be guaranteed to be done
> > > > > > > >> > > fully before the tail is updated. This is not
> > > > > > > >> > > guaranteed with a read barrier, changed to a write
> > > > > > > >> > > barrier just before tail update which in
> > > > > > practice guarantees correct order of reads and writes.
> > > > > > > >> > >
> > > > > > > >> > > Signed-off-by: Juhamatti Kuusisaari
> > > > > > > >> > > <juhamatti.kuusisaari at coriant.com>
> > > > > > > >> >
> > > > > > > >> > Acked-by: Konstantin Ananyev
> > > > > > > >> > <konstantin.ananyev at intel.com>
> > > > > > > >>
> > > > > > > >> Applied, thanks
> > > > > > > >
> > > > > > > > There was ongoing discussion on this
> > > > > > > > http://dpdk.org/ml/archives/dev/2016-July/044168.html
> > > > > > >
> > > > > > > Sorry Jerin, I forgot this email.
> > > > > > > The problem is that nobody replied to your email and you did
> > > > > > > not nack the v2 of this patch.
> > > > >
> > > > > It's probably my bad.
> > > > > I acked the patch before Jerin response, and forgot to reply later.
> > > > >
> > > > > > >
> > > > > > > > This change may not be required as it has the performance impact.
> > > > > > >
> > > > > > > We need to clearly understand what is the performance impact
> > > > > > > (numbers and use cases) on one hand, and is there a real bug
> > > > > > > fixed by this patch on the other hand?
> > > > > >
> > > > > > IHMO, there is no real bug here. rte_smb_rmb() provides the
> > > > > > LOAD-STORE barrier to make sure tail pointer WRITE happens only after prior LOADS.
> > > > >
> > > > > Yep, from what I read at the link Jerin provided, indeed it seems rte_smp_rmb() is enough for the arm arch here...
> > > > > For ppc, as I can see both rte_smp_rmb()/rte_smp_wmb() emits the same instruction.
> > > > >
> > > > > >
> > > > > > Thoughts?
> > > > >
> > > > > Wonder how big is a performance impact?
> > > >
> > > > With this change we need to wait for addtional STORES to be completed to local buffer in addtion to LOADS from ring buffers memory.
> > >
> > > I understand that, just wonder did you see any real performance difference?
> > 
> > Yeah...
> 
> Ok, then I don't see any good reason why we shouldn't revert it.
> I suppose the best way would be to submit a new patch for RC5 to revert the changes.
> Do you prefer to submit it yourself and I'll ack it or visa-versa?

OK. I will submit it then

> Thanks
> Konstantin 
> 
> > 
> > > Probably with ring_perf_autotest/mempool_perf_autotest or something?
> > 
> > W/O change
> > RTE>>ring_perf_autotest
> > ### Testing single element and burst enq/deq ### SP/SC single enq/dequeue: 4 MP/MC single enq/dequeue: 16 SP/SC burst enq/dequeue
> > (size: 8): 0 MP/MC burst enq/dequeue (size: 8): 2 SP/SC burst enq/dequeue (size: 32): 0 MP/MC burst enq/dequeue (size: 32): 0
> > 
> > ### Testing empty dequeue ###
> > SC empty dequeue: 0.35
> > MC empty dequeue: 0.60
> > 
> > ### Testing using a single lcore ###
> > SP/SC bulk enq/dequeue (size: 8): 0.93
> > MP/MC bulk enq/dequeue (size: 8): 2.45
> > SP/SC bulk enq/dequeue (size: 32): 0.58
> > MP/MC bulk enq/dequeue (size: 32): 0.97
> > 
> > ### Testing using two physical cores ### SP/SC bulk enq/dequeue (size: 8): 1.89 MP/MC bulk enq/dequeue (size: 8): 4.28 SP/SC bulk
> > enq/dequeue (size: 32): 0.90 MP/MC bulk enq/dequeue (size: 32): 1.19 Test OK
> > RTE>>
> > 
> > With change
> > RTE>>ring_perf_autotest
> > ### Testing single element and burst enq/deq ### SP/SC single enq/dequeue: 6 MP/MC single enq/dequeue: 16 SP/SC burst enq/dequeue
> > (size: 8): 1 MP/MC burst enq/dequeue (size: 8): 2 SP/SC burst enq/dequeue (size: 32): 0 MP/MC burst enq/dequeue (size: 32): 0
> > 
> > ### Testing empty dequeue ###
> > SC empty dequeue: 0.35
> > MC empty dequeue: 0.60
> > 
> > ### Testing using a single lcore ###
> > SP/SC bulk enq/dequeue (size: 8): 1.28
> > MP/MC bulk enq/dequeue (size: 8): 2.47
> > SP/SC bulk enq/dequeue (size: 32): 0.64
> > MP/MC bulk enq/dequeue (size: 32): 0.97
> > 
> > ### Testing using two physical cores ### SP/SC bulk enq/dequeue (size: 8): 2.08 MP/MC bulk enq/dequeue (size: 8): 4.29 SP/SC bulk
> > enq/dequeue (size: 32): 1.24 MP/MC bulk enq/dequeue (size: 32): 1.19 Test OK
> > 
> > > Konstantin
> > >
> > > >
> > > > > If there is a real one, I suppose we can revert the patch?
> > > >
> > > > Request to revert this one as their no benifts for other
> > > > architectures and indeed it creates addtional delay in waiting for STORES to complete in ARM.
> > > > Lets do the correct thing by reverting it.
> > > >
> > > > Jerin
> > > >
> > > >
> > > >
> > > > > Konstantin
> > > > >
> > > > > >
> > > > > > >
> > > > > > > Please guys make things clear and we'll revert if needed.