Re: FW: [PATCH v4 3/3] mempool: use cache for frequently updated stats

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Nov 08, 2022 at 04:51:11PM +0100, Thomas Monjalon wrote:
> 08/11/2022 15:30, Morten Brørup:
> > > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > > 08/11/2022 12:25, Morten Brørup:
> > > > From: Morten Brørup
> > > > > From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com]
> > > > > Sent: Tuesday, 8 November 2022 10.20
> > > > > > +#ifdef RTE_LIBRTE_MEMPOOL_STATS
> > > > > > +#define RTE_MEMPOOL_CACHE_STAT_ADD(cache, name, n) (cache)-
> > > >stats.name += n
> > > > >
> > > > > As Andrew already pointed, it needs to be: ((cache)->stats.name +=
> > > (n))
> > > > > Apart from that, LGTM.
> > > > > Series-Acked-by: Konstantin Ananyev <konstantin.ananyev@huawei.com>
> > > >
> > > > @Thomas, this series should be ready to apply... it now has been:
> > > > Reviewed-by: (mempool maintainer) Andrew Rybchenko
> > > <andrew.rybchenko@oktetlabs.ru>
> > > > Reviewed-By: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> > > > Acked-by: Konstantin Ananyev <konstantin.ananyev@huawei.com>
> > > 
> > > Being acked does not mean it is good to apply in -rc3.
> > 
> > I understand that the RFC/v1 of this series was formally too late to make it in 22.11, so I will not complain loudly if you choose to omit it for 22.11.
> > 
> > With two independent reviews, including from a mempool maintainer, I still have some hope. Also considering the risk assessment below. ;-)
> > 
> > > Please tell what is the benefit for 22.11 (before/after and condition).
> > 
> > Short version: With this series, mempool statistics can be used in production. Without it, the performance cost (mempool_perf_autotest: -74 %) is prohibitive!
> > 
> > Long version:
> > 
> > The patch series provides significantly higher performance for mempool statistics, which are readable through rte_mempool_dump(FILE *f, struct rte_mempool *mp).
> > 
> > Without this series, you have to set RTE_LIBRTE_MEMPOOL_DEBUG at build time to get mempool statistics. RTE_LIBRTE_MEMPOOL_DEBUG also enables protective cookies before and after each mempool object, which are all verified on get/put from the mempool. According to mempool_perf_autotest, the performance cost of mempool statistics (by setting RTE_LIBRTE_MEMPOOL_DEBUG) is a 74 % decrease in rate_persec for mempools with cache (i.e. mbuf pools). Prohibitive for use in production!
> > 
> > With this series, the performance cost of mempool statistics (by setting RTE_LIBRTE_MEMPOOL_STATS) in mempool_perf_autotest is only 6.7 %, so mempool statistics can be used in production.
> > 
> > > Note there is a real risk doing such change that late.
> > 
> > Risk assessment:
> > 
> > The patch series has zero effect unless either RTE_LIBRTE_MEMPOOL_DEBUG or RTE_LIBRTE_MEMPOOL_STATS are set when building. They are not set in the default build.
> 
> If theses build flags are not set, there is no risk and no benefit.
> But if they are set, there is a risk of regression,
> for the benefit of an increased performance of a debug feature.
> I would say it is better to avoid any functional regression in a debug feature
> at this stage.
> Any other opinion?
> 
While I agree that we should avoid any functional regression, I wonder how
widely used the debug feature is, and how big the risk of a regression is?
Even if there is one, having a regression in a debug feature is a lot less
serious than having one in something which goes into production.

/Bruce