[PATCH v5] mempool cache: add zero-copy get and put functions

[Kamalakshitha Aligeri]

-----Original Message-----
From: Bruce Richardson <bruce.richardson at intel.com> 
Sent: Monday, January 23, 2023 6:31 AM
To: Morten Brørup <mb at smartsharesystems.com>
Cc: Konstantin Ananyev <konstantin.ananyev at huawei.com>; Konstantin Ananyev <konstantin.v.ananyev at yandex.ru>; olivier.matz at 6wind.com; andrew.rybchenko at oktetlabs.ru; Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>; Kamalakshitha Aligeri <Kamalakshitha.Aligeri at arm.com>; dev at dpdk.org; nd <nd at arm.com>
Subject: Re: [PATCH v5] mempool cache: add zero-copy get and put functions

On Mon, Jan 23, 2023 at 01:23:50PM +0100, Morten Br�rup wrote:
> > From: Konstantin Ananyev [mailto:konstantin.ananyev at huawei.com]
> > Sent: Monday, 23 January 2023 12.54
> > 
> > > > Few nits, see below.
> > > > Also I still think we do need a test case for _zc_get_ before 
> > > > accepting it in the mainline.
> > >
I am working on the test cases. Will submit it soon

> > > Poking at my bad conscience... :-)
> > >
> > > It's on my todo-list. Apparently not high enough. ;-)
> > >
> > > > With that in place:
> > > > Acked-by: Konstantin Ananyev <konstantin.v.ananyev at yandex.ru>
> > > >
> 
> [...]
> 
> > > > > +/**
> > > > > + * @warning
> > > > > + * @b EXPERIMENTAL: This API may change, or be removed, 
> > > > > +without
> > > > prior notice.
> > > > > + *
> > > > > + * Zero-copy put objects in a user-owned mempool cache backed 
> > > > > + by
> > the
> > > > specified mempool.
> > > > > + *
> > > > > + * @param cache
> > > > > + *   A pointer to the mempool cache.
> > > > > + * @param mp
> > > > > + *   A pointer to the mempool.
> > > > > + * @param n
> > > > > + *   The number of objects to be put in the mempool cache.
> > > > > + * @return
> > > > > + *   The pointer to where to put the objects in the mempool
> > cache.
> > > > > + *   NULL if the request itself is too big for the cache, i.e.
> > > > > + *   exceeds the cache flush threshold.
> > > > > + */
> > > > > +__rte_experimental
> > > > > +static __rte_always_inline void ** 
> > > > > +rte_mempool_cache_zc_put_bulk(struct rte_mempool_cache *cache,
> > > > > +		struct rte_mempool *mp,
> > > > > +		unsigned int n)
> > > > > +{
> > > > > +	RTE_ASSERT(cache != NULL);
> > > > > +	RTE_ASSERT(mp != NULL);
> > > > > +
> > > > > +	rte_mempool_trace_cache_zc_put_bulk(cache, mp, n);
> > > > > +	return __rte_mempool_cache_zc_put_bulk(cache, mp, n); }
> > > > > +
> > > > > +/**
> > > > > + * @warning
> > > > > + * @b EXPERIMENTAL: This API may change, or be removed, 
> > > > > +without
> > > > prior notice.
> > > > > + *
> > > > > + * Zero-copy un-put objects in a user-owned mempool cache.
Why is it written as user-owned mempool cache. API expects a pointer to mempool cache right, whether it is default or user-owned does not make any difference
Please correct me if I am wrong
> > > > > + *
> > > > > + * @param cache
> > > > > + *   A pointer to the mempool cache.
> > > > > + * @param n
> > > > > + *   The number of objects not put in the mempool cache after
> > > > calling
> > > > > + *   rte_mempool_cache_zc_put_bulk().
> > > > > + */
> > > > > +__rte_experimental
> > > > > +static __rte_always_inline void 
> > > > > +rte_mempool_cache_zc_put_rewind(struct rte_mempool_cache *cache,
> > > > > +		unsigned int n)
> > > > > +{
> > > > > +	RTE_ASSERT(cache != NULL);
> > > > > +	RTE_ASSERT(n <= cache->len);
> > > > > +
> > > > > +	rte_mempool_trace_cache_zc_put_rewind(cache, n);
> > > > > +
> > > > > +	cache->len -= n;
> > > > > +
> > > > > +	RTE_MEMPOOL_CACHE_STAT_ADD(cache, put_objs, (int)-n); }
> > > > > +
> > > > > +/**
> > > > > + * @warning
> > > > > + * @b EXPERIMENTAL: This API may change, or be removed, 
> > > > > +without
> > > > prior notice.
> > > > > + *
> > > > > + * Zero-copy get objects from a user-owned mempool cache 
> > > > > + backed
> > by
> > > > the specified mempool.
> > > > > + *
> > > > > + * @param cache
> > > > > + *   A pointer to the mempool cache.
> > > > > + * @param mp
> > > > > + *   A pointer to the mempool.
> > > > > + * @param n
> > > > > + *   The number of objects to prefetch into the mempool cache.
> > > >
> > > > Why not 'get' instead of 'prefetch'?
> > >
> > > This was my thinking:
> > >
> > > The function "prefetches" the objects into the cache. It is the
> > application itself that "gets" the objects from the cache after 
> > having
> > > called the function.
> > > You might also notice that the n parameter for the zc_put() 
> > > function
> > is described as "to be put" (future), not "to put" (now) in the
> > > cache.
> > >
> > > On the other hand, I chose "Zero-copy get" for the function 
> > > headline
> > to keep it simple.
> > >
> > > If you think "get" is a more correct description of the n 
> > > parameter,
> > I can change it.
> > >
> > > Alternatively, I can use the same style as zc_put(), i.e. "to be
> > gotten from the mempool cache" - but that would require input from a
> > > natively English speaking person, because Danish and English 
> > > grammar
> > is very different, and I am highly uncertain about my English
> > > grammar here! I originally considered this phrase, but concluded 
> > > that
> > the "prefetch" description was easier to understand - especially
> > > for non-native English readers.
> > 
> > For me 'prefetch' seems a bit unclear in that situation...
> > Probably: "number of objects that user plans to extract from the 
> > cache"?
> > But again, I am not native English speaker too, so might be someone 
> > can suggest a better option.
> > 
> 
> @Bruce (or any other native English speaking person), your input would be appreciated here!
> 
I was happily ignoring this thread until you went and dragged me in with a hard question. :-)

I think the longer explanation the clearer it is likely to be. How about "number of objects to be made available for extraction from the cache"? I don't like the reference to "the user" in the longer suggestion above, but otherwise consider it clearer that talking of prefetching or "getting".

My 2c.

/Bruce