[dpdk-dev] [PATCH v3] mempool: remove non-EAL thread note from header

[Eads, Gage]

>  -----Original Message-----
>  From: Olivier Matz [mailto:olivier.matz at 6wind.com]
>  Sent: Friday, March 24, 2017 9:05 AM
>  To: Eads, Gage <gage.eads at intel.com>
>  Cc: dev at dpdk.org
>  Subject: Re: [PATCH v3] mempool: remove non-EAL thread note from header
>  
>  Hi Gage,
>  
>  On Thu, 23 Mar 2017 09:20:58 -0500, Gage Eads <gage.eads at intel.com>
>  wrote:
>  > Commit 30e6399892276 ("mempool: support non-EAL thread") added the
>  > capability for non-EAL threads to use the mempool library. This commit
>  > removes the note indicating that the mempool library cannot be used
>  > safely by non-EAL threads.
>  >
>  > Also, fix a typo.
>  >
>  > Signed-off-by: Gage Eads <gage.eads at intel.com>
>  > ---
>  > v2: Changed commit message to referenced commit 30e63998 instead of
>  > 4b5062755
>  > v3: Fix checkpatch error
>  >
>  >  lib/librte_mempool/rte_mempool.h | 6 +-----
>  >  1 file changed, 1 insertion(+), 5 deletions(-)
>  >
>  > diff --git a/lib/librte_mempool/rte_mempool.h
>  > b/lib/librte_mempool/rte_mempool.h
>  > index 991feaa..b1186fd 100644
>  > --- a/lib/librte_mempool/rte_mempool.h
>  > +++ b/lib/librte_mempool/rte_mempool.h
>  > @@ -53,11 +53,7 @@
>  >   *
>  >   * Note: the mempool implementation is not preemptable. A lcore must
>  >   * not be interrupted by another task that uses the same mempool
>  > - * (because it uses a ring which is not preemptable). Also, mempool
>  > - * functions must not be used outside the DPDK environment: for
>  > - * example, in linuxapp environment, a thread that is not created by
>  > - * the EAL must not use mempools. This is due to the per-lcore cache
>  > - * that won't work as rte_lcore_id() will not return a correct value.
>  > + * (because it uses a ring which is not preemptible).
>  >   */
>  >
>  >  #include <stdio.h>
>  
>  I agree the comment is not correct today. But I think we should still highlight
>  that usual functions [ex: rte_mempool_get(), rte_mempool_put()] won't work
>  when called on a non-EAL thread.

My understanding is that non-EAL threads can call those functions, but their performance will suffer since they go directly to the underlying ring. Is that correct?

>  
>  Maybe it could be reworded in that way:
>  
>  """
>  Note: the mempool implementation is not preemptable. A lcore must not be
>  interrupted by another task that uses the same mempool (because it uses a ring
>  which is not preemptible). Also, usual mempool functions like
>  rte_mempool_get() or rte_mempool_put() are designed to be called from an
>  EAL thread due to the internal per-lcore cache. When using a mempool from a
>  non-EAL thread, a user cache has to be provided to
>  rte_mempool_generic_get() or rte_mempool_generic_put().
>  """
>  
>  What do you think?

Agreed, this looks better. If indeed non-EAL threads can call rte_mempool_get() and _put(), perhaps the last sentence can be reworked like so?

Due to the lack of caching, rte_mempool_get() or rte_mempool_put() performance
will suffer when called by non-EAL threads. Instead, non-EAL threads should call
rte_mempool_generic_get() or rte_mempool_generic_put() with a user cache
created with rte_mempool_cache_create().

>  
>  Thanks,
>  Olivier