Re: [PATCH] app/dma-perf: replace pktmbuf with mempool objects

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

On Tue, Dec 12, 2023 at 04:16:20PM +0100, Morten Brørup wrote:
> +TO: Bruce, please stop me if I'm completely off track here.
> 
> > From: Ferruh Yigit [mailto:ferruh.yigit@amd.com] Sent: Tuesday, 12
> > December 2023 15.38
> > 
> > On 12/12/2023 11:40 AM, Morten Brørup wrote:
> > >> From: Vipin Varghese [mailto:vipin.varghese@amd.com] Sent: Tuesday,
> > >> 12 December 2023 11.38
> > >>
> > >> Replace pktmbuf pool with mempool, this allows increase in MOPS
> > >> especially in lower buffer size. Using Mempool, allows to reduce the
> > >> extra CPU cycles.
> > >
> > > I get the point of this change: It tests the performance of copying
> > raw memory objects using respectively rte_memcpy and DMA, without the
> > mbuf indirection overhead.
> > >
> > > However, I still consider the existing test relevant: The performance
> > of copying packets using respectively rte_memcpy and DMA.
> > >
> > 
> > This is DMA performance test application and packets are not used,
> > using pktmbuf just introduces overhead to the main focus of the
> > application.
> > 
> > I am not sure if pktmuf selected intentionally for this test
> > application, but I assume it is there because of historical reasons.
> 
> I think pktmbuf was selected intentionally, to provide more accurate
> results for application developers trying to determine when to use
> rte_memcpy and when to use DMA. Much like the "copy breakpoint" in Linux
> Ethernet drivers is used to determine which code path to take for each
> received packet.
> 
> Most applications will be working with pktmbufs, so these applications
> will also experience the pktmbuf overhead. Performance testing with the
> same overhead as the application will be better to help the application
> developer determine when to use rte_memcpy and when to use DMA when
> working with pktmbufs.
> 
> (Furthermore, for the pktmbuf tests, I wonder if copying performance
> could also depend on IOVA mode and RTE_IOVA_IN_MBUF.)
> 
> Nonetheless, there may also be use cases where raw mempool objects are
> being copied by rte_memcpy or DMA, so adding tests for these use cases
> are useful.
> 
> 
> @Bruce, you were also deeply involved in the DMA library, and probably
> have more up-to-date practical experience with it. Am I right that
> pktmbuf overhead in these tests provides more "real life use"-like
> results? Or am I completely off track with my thinking here, i.e. the
> pktmbuf overhead is only noise?
> 
I'm actually not that familiar with the dma-test application, so can't
comment on the specific overhead involved here. In the general case, if we
are just talking about the overhead of dereferencing the mbufs then I would
expect the overhead to be negligible. However, if we are looking to include
the cost of allocation and freeing of buffers, I'd try to avoid that as it
is a cost that would have to be paid for both SW copies and HW copies, so
should not count when calculating offload cost.

/Bruce