Re: [dpdk-dev] [PATCH v3 08/12] mempool: allow config override on element alignment

[Tony Lu <zlu@ezchip.com>]

>-----Original Message-----
>From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce Richardson
>Sent: Monday, July 06, 2015 11:38 PM
>To: Zhigang Lu
>Cc: dev@dpdk.org
>Subject: Re: [dpdk-dev] [PATCH v3 08/12] mempool: allow config override on
>element alignment
>
>On Mon, Jul 06, 2015 at 04:51:33PM +0800, Zhigang Lu wrote:
>> On TILE-Gx and TILE-Mx platforms, the buffers fed into the hardware
>> buffer manager require a 128-byte alignment.  With this change, we
>> allow configuration based override of the element alignment, and
>> default to RTE_CACHE_LINE_SIZE if left unspecified.
>>
>> Change-Id: I9cd789d92b0bc9c8f44a633de59bb04d45d927a7
>> Signed-off-by: Zhigang Lu <zlu@ezchip.com>
>
>This looks an OK change. However, would it be worthwhile making this a
runtime
>parameter rather than a compile-time one? Is it likely that we will ever
have a
>case where someone wants two mempools with different alignments (and
>where using the larger of the two would be problematic)?

For now, I don't think it is very much worthwhile making this a runtime
parameter,
since doing so requires changing the mempool library API, and also the users
of mempool
do not quite care about the underlying alignments. Currently, the alignment
for mempool
objects is mostly a hardware requirement (currently RTE_CACHE_LINE_SIZE for
good
performance). And now we are defining a new RTE_MEMPOOL_ALIGN for mempool
alignment requirement for cases where someone needs other alignments than
RTE_CACHE_LINE_SIZE.

If someone wants two mempools with different alignments, using the larger
one would
not be a problem in current mempool implementation. Because even for the
case where
there is one alignment requirement RTE_CACHE_LINE_SIZE, we would provide
larger one
(2* RTE_CACHE_LINE_SIZE and larger) for allocated mempool objects, as those
objects
are continuous in memory. So we could not avoid larger one in current
implementation.

Thanks again for reviewing!
-Zhigang

>/Bruce