Re: [dpdk-dev] [RFC PATCH 02/13] add vhost packed ring fast enqueue function

["Liu, Yong" <yong.liu@intel.com>]

> -----Original Message-----
> From: Jason Wang [mailto:jasowang@redhat.com]
> Sent: Thursday, July 11, 2019 12:11 PM
> To: Liu, Yong <yong.liu@intel.com>; Bie, Tiwei <tiwei.bie@intel.com>;
> maxime.coquelin@redhat.com; dev@dpdk.org
> Subject: Re: [dpdk-dev] [RFC PATCH 02/13] add vhost packed ring fast enqueue
> function
> 
> 
> On 2019/7/10 下午3:30, Liu, Yong wrote:
> >
> >> -----Original Message-----
> >> From: Jason Wang [mailto:jasowang@redhat.com]
> >> Sent: Wednesday, July 10, 2019 12:28 PM
> >> To: Liu, Yong <yong.liu@intel.com>; Bie, Tiwei <tiwei.bie@intel.com>;
> >> maxime.coquelin@redhat.com; dev@dpdk.org
> >> Subject: Re: [dpdk-dev] [RFC PATCH 02/13] add vhost packed ring fast
> enqueue
> >> function
> >>
> >>
> >> On 2019/7/9 上午1:13, Marvin Liu wrote:
> >>> In fast enqueue function, will first check whether descriptors are
> >>> cache aligned. Fast enqueue function will check prerequisites in the
> >>> beginning. Fast enqueue function do not support chained mbufs, normal
> >>> function will handle that.
> >>>
> >>> Signed-off-by: Marvin Liu <yong.liu@intel.com>
> >> Any reason for not letting compiler to unroll the loops?
> >>
> > Hi Jason,
> > I'm not sure about how much compiler can help on unrolling loops as it
> can't know how much loops will create in one call.
> > After force not using unroll-loop optimization by "-fno-unroll-loops",
> virtio_dev_rx_packed function size remained the same.
> > So look like gcc unroll-loop optimization do not help here.
> 
> 
> I meant something like "pragma GCC unroll N" just before the loop you
> want unrolled.
> 
> Thanks
> 

Hi Jason,
Just tired with gcc8.3.0 and master code, only 0.1Mpps performance gain with "#pragma GCC unroll". 
I think this compiler pragma is not helpful in the big loop which contained so much functions. 

Thanks,
Marvin

> 
> >
> > And fast enqueue function not only did unroll loop, it also checked cache
> alignment which can help performance in another side.
> >
> > Regards,
> > Marvin
> >
> >> Thanks
> >>