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Re: [PATCH 1/3] ptr_ring: batch ring zeroing

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  Re: [PATCH 1/3] ptr_ring: batch ring zeroing Jason Wang <jasowang@redhat.com> - 2017-04-12 10:10 +0200
    Re: [PATCH 1/3] ptr_ring: batch ring zeroing Jason Wang <jasowang@redhat.com> - 2017-04-14 10:00 +0200
      Re: [PATCH 1/3] ptr_ring: batch ring zeroing "Michael S. Tsirkin" <mst@redhat.com> - 2017-04-14 23:10 +0200
        Re: [PATCH 1/3] ptr_ring: batch ring zeroing Jason Wang <jasowang@redhat.com> - 2017-04-18 04:20 +0200
      Re: [PATCH 1/3] ptr_ring: batch ring zeroing "Michael S. Tsirkin" <mst@redhat.com> - 2017-04-15 01:00 +0200
        Re: [PATCH 1/3] ptr_ring: batch ring zeroing Jason Wang <jasowang@redhat.com> - 2017-04-18 04:20 +0200

#1621902 — Re: [PATCH 1/3] ptr_ring: batch ring zeroing

On 2017年04月07日 13:49, Michael S. Tsirkin wrote:
> A known weakness in ptr_ring design is that it does not handle well the
> situation when ring is almost full: as entries are consumed they are
> immediately used again by the producer, so consumer and producer are
> writing to a shared cache line.
>
> To fix this, add batching to consume calls: as entries are
> consumed do not write NULL into the ring until we get
> a multiple (in current implementation 2x) of cache lines
> away from the producer. At that point, write them all out.
>
> We do the write out in the reverse order to keep
> producer from sharing cache with consumer for as long
> as possible.
>
> Writeout also triggers when ring wraps around - there's
> no special reason to do this but it helps keep the code
> a bit simpler.
>
> What should we do if getting away from producer by 2 cache lines
> would mean we are keeping the ring moe than half empty?
> Maybe we should reduce the batching in this case,
> current patch simply reduces the batching.
>
> Notes:
> - it is no longer true that a call to consume guarantees
>    that the following call to produce will succeed.
>    No users seem to assume that.
> - batching can also in theory reduce the signalling rate:
>    users that would previously send interrups to the producer
>    to wake it up after consuming each entry would now only
>    need to do this once in a batch.
>    Doing this would be easy by returning a flag to the caller.
>    No users seem to do signalling on consume yet so this was not
>    implemented yet.
>
> Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
> ---
>
> Jason, I am curious whether the following gives you some of
> the performance boost that you see with vhost batching
> patches. Is vhost batching on top still helpful?

The patch looks good to me, will have a test for vhost batching patches.

Thanks

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#1623550

On 2017年04月12日 16:03, Jason Wang wrote:
>
>
> On 2017年04月07日 13:49, Michael S. Tsirkin wrote:
>> A known weakness in ptr_ring design is that it does not handle well the
>> situation when ring is almost full: as entries are consumed they are
>> immediately used again by the producer, so consumer and producer are
>> writing to a shared cache line.
>>
>> To fix this, add batching to consume calls: as entries are
>> consumed do not write NULL into the ring until we get
>> a multiple (in current implementation 2x) of cache lines
>> away from the producer. At that point, write them all out.
>>
>> We do the write out in the reverse order to keep
>> producer from sharing cache with consumer for as long
>> as possible.
>>
>> Writeout also triggers when ring wraps around - there's
>> no special reason to do this but it helps keep the code
>> a bit simpler.
>>
>> What should we do if getting away from producer by 2 cache lines
>> would mean we are keeping the ring moe than half empty?
>> Maybe we should reduce the batching in this case,
>> current patch simply reduces the batching.
>>
>> Notes:
>> - it is no longer true that a call to consume guarantees
>>    that the following call to produce will succeed.
>>    No users seem to assume that.
>> - batching can also in theory reduce the signalling rate:
>>    users that would previously send interrups to the producer
>>    to wake it up after consuming each entry would now only
>>    need to do this once in a batch.
>>    Doing this would be easy by returning a flag to the caller.
>>    No users seem to do signalling on consume yet so this was not
>>    implemented yet.
>>
>> Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
>> ---
>>
>> Jason, I am curious whether the following gives you some of
>> the performance boost that you see with vhost batching
>> patches. Is vhost batching on top still helpful?
>
> The patch looks good to me, will have a test for vhost batching patches.
>
> Thanks

Still helpful:

before this patch: 1.84Mpps
with this patch: 2.00Mpps
with batch dequeuing: 2.30Mpps

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks

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#1623916

On Fri, Apr 14, 2017 at 03:52:23PM +0800, Jason Wang wrote:
> 
> 
> On 2017年04月12日 16:03, Jason Wang wrote:
> > 
> > 
> > On 2017年04月07日 13:49, Michael S. Tsirkin wrote:
> > > A known weakness in ptr_ring design is that it does not handle well the
> > > situation when ring is almost full: as entries are consumed they are
> > > immediately used again by the producer, so consumer and producer are
> > > writing to a shared cache line.
> > > 
> > > To fix this, add batching to consume calls: as entries are
> > > consumed do not write NULL into the ring until we get
> > > a multiple (in current implementation 2x) of cache lines
> > > away from the producer. At that point, write them all out.
> > > 
> > > We do the write out in the reverse order to keep
> > > producer from sharing cache with consumer for as long
> > > as possible.
> > > 
> > > Writeout also triggers when ring wraps around - there's
> > > no special reason to do this but it helps keep the code
> > > a bit simpler.
> > > 
> > > What should we do if getting away from producer by 2 cache lines
> > > would mean we are keeping the ring moe than half empty?
> > > Maybe we should reduce the batching in this case,
> > > current patch simply reduces the batching.
> > > 
> > > Notes:
> > > - it is no longer true that a call to consume guarantees
> > >    that the following call to produce will succeed.
> > >    No users seem to assume that.
> > > - batching can also in theory reduce the signalling rate:
> > >    users that would previously send interrups to the producer
> > >    to wake it up after consuming each entry would now only
> > >    need to do this once in a batch.
> > >    Doing this would be easy by returning a flag to the caller.
> > >    No users seem to do signalling on consume yet so this was not
> > >    implemented yet.
> > > 
> > > Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
> > > ---
> > > 
> > > Jason, I am curious whether the following gives you some of
> > > the performance boost that you see with vhost batching
> > > patches. Is vhost batching on top still helpful?
> > 
> > The patch looks good to me, will have a test for vhost batching patches.
> > 
> > Thanks
> 
> Still helpful:
> 
> before this patch: 1.84Mpps
> with this patch: 2.00Mpps
> with batch dequeuing: 2.30Mpps
> 
> Acked-by: Jason Wang <jasowang@redhat.com>
> 
> Thanks

Fascinating. How do we explain the gain with batch dequeue?
Is it just the lock overhead? Can you pls try to replace
the lock with a simple non-fair atomic and see what happens?

-- 
MST

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#1624991

On 2017年04月15日 05:00, Michael S. Tsirkin wrote:
> On Fri, Apr 14, 2017 at 03:52:23PM +0800, Jason Wang wrote:
>>
>> On 2017年04月12日 16:03, Jason Wang wrote:
>>>
>>> On 2017年04月07日 13:49, Michael S. Tsirkin wrote:
>>>> A known weakness in ptr_ring design is that it does not handle well the
>>>> situation when ring is almost full: as entries are consumed they are
>>>> immediately used again by the producer, so consumer and producer are
>>>> writing to a shared cache line.
>>>>
>>>> To fix this, add batching to consume calls: as entries are
>>>> consumed do not write NULL into the ring until we get
>>>> a multiple (in current implementation 2x) of cache lines
>>>> away from the producer. At that point, write them all out.
>>>>
>>>> We do the write out in the reverse order to keep
>>>> producer from sharing cache with consumer for as long
>>>> as possible.
>>>>
>>>> Writeout also triggers when ring wraps around - there's
>>>> no special reason to do this but it helps keep the code
>>>> a bit simpler.
>>>>
>>>> What should we do if getting away from producer by 2 cache lines
>>>> would mean we are keeping the ring moe than half empty?
>>>> Maybe we should reduce the batching in this case,
>>>> current patch simply reduces the batching.
>>>>
>>>> Notes:
>>>> - it is no longer true that a call to consume guarantees
>>>>     that the following call to produce will succeed.
>>>>     No users seem to assume that.
>>>> - batching can also in theory reduce the signalling rate:
>>>>     users that would previously send interrups to the producer
>>>>     to wake it up after consuming each entry would now only
>>>>     need to do this once in a batch.
>>>>     Doing this would be easy by returning a flag to the caller.
>>>>     No users seem to do signalling on consume yet so this was not
>>>>     implemented yet.
>>>>
>>>> Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
>>>> ---
>>>>
>>>> Jason, I am curious whether the following gives you some of
>>>> the performance boost that you see with vhost batching
>>>> patches. Is vhost batching on top still helpful?
>>> The patch looks good to me, will have a test for vhost batching patches.
>>>
>>> Thanks
>> Still helpful:
>>
>> before this patch: 1.84Mpps
>> with this patch: 2.00Mpps
>> with batch dequeuing: 2.30Mpps
>>
>> Acked-by: Jason Wang <jasowang@redhat.com>
>>
>> Thanks
> Fascinating. How do we explain the gain with batch dequeue?

I count the drop rate (pktgen on tun and count tun tx) and maybe it can 
explain more or less:

Before this patch: TX xmit 1.8Mpps Tx dropped 0.23Mpps Tx total 2.04Mpps 
11% dropped
After this patch: Tx xmit 1.95Mpps Tx dropped 0.33Mpps Tx total 2.28Mpps 
14% dropped
With batched dequeuing: Tx xmit 2.24Mpps Tx dropped 0.01Mpps Tx total 
2.26Mpps ~0% dropped

With this patch, we remove the cache contention by blocking the producer 
more or less. With batch dequeuing, the ring is not full in 99% of the 
cases which probably means the producer is not blocked for most of the time.

> Is it just the lock overhead?

I remove the spinlocks for peeking and dequeuing on top of this patch. 
The tx pps were increased from ~2Mpps to ~2.08Mpps. So it was not only 
the lock overhead.

> Can you pls try to replace
> the lock with a simple non-fair atomic and see what happens?
>

Not sure I get the idea, we are going for fast path of spinlocks for 
sure which is just a cmpxchg().

Thanks

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#1623949

On Fri, Apr 14, 2017 at 03:52:23PM +0800, Jason Wang wrote:
> 
> 
> On 2017年04月12日 16:03, Jason Wang wrote:
> > 
> > 
> > On 2017年04月07日 13:49, Michael S. Tsirkin wrote:
> > > A known weakness in ptr_ring design is that it does not handle well the
> > > situation when ring is almost full: as entries are consumed they are
> > > immediately used again by the producer, so consumer and producer are
> > > writing to a shared cache line.
> > > 
> > > To fix this, add batching to consume calls: as entries are
> > > consumed do not write NULL into the ring until we get
> > > a multiple (in current implementation 2x) of cache lines
> > > away from the producer. At that point, write them all out.
> > > 
> > > We do the write out in the reverse order to keep
> > > producer from sharing cache with consumer for as long
> > > as possible.
> > > 
> > > Writeout also triggers when ring wraps around - there's
> > > no special reason to do this but it helps keep the code
> > > a bit simpler.
> > > 
> > > What should we do if getting away from producer by 2 cache lines
> > > would mean we are keeping the ring moe than half empty?
> > > Maybe we should reduce the batching in this case,
> > > current patch simply reduces the batching.
> > > 
> > > Notes:
> > > - it is no longer true that a call to consume guarantees
> > >    that the following call to produce will succeed.
> > >    No users seem to assume that.
> > > - batching can also in theory reduce the signalling rate:
> > >    users that would previously send interrups to the producer
> > >    to wake it up after consuming each entry would now only
> > >    need to do this once in a batch.
> > >    Doing this would be easy by returning a flag to the caller.
> > >    No users seem to do signalling on consume yet so this was not
> > >    implemented yet.
> > > 
> > > Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
> > > ---
> > > 
> > > Jason, I am curious whether the following gives you some of
> > > the performance boost that you see with vhost batching
> > > patches. Is vhost batching on top still helpful?
> > 
> > The patch looks good to me, will have a test for vhost batching patches.
> > 
> > Thanks
> 
> Still helpful:
> 
> before this patch: 1.84Mpps
> with this patch: 2.00Mpps
> with batch dequeuing: 2.30Mpps

Just a thought: could you test dropping the consumer spinlock
completely?  Just around the peek?

As I said previously, perf c2c tool should be helpful
to locate sources latency related to cache.

> Acked-by: Jason Wang <jasowang@redhat.com>
> 
> Thanks

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#1624990

On 2017年04月15日 06:50, Michael S. Tsirkin wrote:
> On Fri, Apr 14, 2017 at 03:52:23PM +0800, Jason Wang wrote:
>>
>> On 2017年04月12日 16:03, Jason Wang wrote:
>>>
>>> On 2017年04月07日 13:49, Michael S. Tsirkin wrote:
>>>> A known weakness in ptr_ring design is that it does not handle well the
>>>> situation when ring is almost full: as entries are consumed they are
>>>> immediately used again by the producer, so consumer and producer are
>>>> writing to a shared cache line.
>>>>
>>>> To fix this, add batching to consume calls: as entries are
>>>> consumed do not write NULL into the ring until we get
>>>> a multiple (in current implementation 2x) of cache lines
>>>> away from the producer. At that point, write them all out.
>>>>
>>>> We do the write out in the reverse order to keep
>>>> producer from sharing cache with consumer for as long
>>>> as possible.
>>>>
>>>> Writeout also triggers when ring wraps around - there's
>>>> no special reason to do this but it helps keep the code
>>>> a bit simpler.
>>>>
>>>> What should we do if getting away from producer by 2 cache lines
>>>> would mean we are keeping the ring moe than half empty?
>>>> Maybe we should reduce the batching in this case,
>>>> current patch simply reduces the batching.
>>>>
>>>> Notes:
>>>> - it is no longer true that a call to consume guarantees
>>>>     that the following call to produce will succeed.
>>>>     No users seem to assume that.
>>>> - batching can also in theory reduce the signalling rate:
>>>>     users that would previously send interrups to the producer
>>>>     to wake it up after consuming each entry would now only
>>>>     need to do this once in a batch.
>>>>     Doing this would be easy by returning a flag to the caller.
>>>>     No users seem to do signalling on consume yet so this was not
>>>>     implemented yet.
>>>>
>>>> Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
>>>> ---
>>>>
>>>> Jason, I am curious whether the following gives you some of
>>>> the performance boost that you see with vhost batching
>>>> patches. Is vhost batching on top still helpful?
>>> The patch looks good to me, will have a test for vhost batching patches.
>>>
>>> Thanks
>> Still helpful:
>>
>> before this patch: 1.84Mpps
>> with this patch: 2.00Mpps
>> with batch dequeuing: 2.30Mpps
> Just a thought: could you test dropping the consumer spinlock
> completely?  Just around the peek?

2% improvement for dropping spinlock around peeking, 2% more for 
dropping spinlock for consuming.

>
> As I said previously, perf c2c tool should be helpful
> to locate sources latency related to cache.
>

perf c2c indeeds shows some false sharing were reduced by this patch. 
But it does not show obvious different with batch dequeuing on top.

Thanks

>> Acked-by: Jason Wang <jasowang@redhat.com>
>>
>> Thanks

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