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

["Morten Brørup" <mb@smartsharesystems.com>]

+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?

> 
> 
> > So, instead of replacing mbufs with mempool objects, please add new
> test variants using mempool objects.
> >