Cortex-A35 performance for DDR3 read accesses

Hi,

 

I am using iMX 8X which has 1 cluster of 4 Cortex-A35 cores, with DDR3L (DDR3-1866) with ECC enabled.

I performed some measurement for MEMCPY and MEMSET functions to have an estimate of the DDR bandwidth, with one cortex-A35 core running. Here are the best results I have:

- MEMSET: 6079 MB/s

- MEMCPY: 2081 MB/s

- MEMREAD: 2880 MB/s

 

The functions are based on NEON instructions with prefetch memory instructions (except MEMSET which has no prefetch memory instructions), and caches and MMU are active.

The idea here is to configure the core or the cluster components to get as close as possible to the theoretical bandwidth, which is 7464MB/s (DDR-1866, 32 bits), in order to fasten code execution from DDR for a normal application running on 1 Cortex-A35 core.

As the MEMSET measured bandwidth seems acceptable (81% of theoretical bandwidth), it would be surprising if read accesses were not optimizable.

According to read access latency of the used DDR chip (13 cycles) and write access latency (9 cycles), I would have expected a difference between MEMSET and MEMREAD functions, but not as much, especially because dur to MMU and caches activation, I would expect the controller to perform continuous accesses to the DDR, where the read latency and write latency impact is minimized.

Though I have already posted some questions about the DDR controller of the iMX 8X on NXP forum, I also tried different settings in the Cortex-A35 to try to optimize the read accesses, but I can't get significant improvements:

  • CPUACTLR.DTAH and LDP instructions
  • CPUACTLR.L1PCTL

In some discussions on iMX forums, I also found that using 4 cores instead of 1 also enhance the bandwidth available, by 10%-20%.

Using the caches and MMU or not impacts directly the memory tests results (because cache lines are filled in background), and I am convinced that there are still certain things that I should understand to be able to configure the core correctly, but I can't find what.

Does anyone has information on:

  1. Though the MEMSET result seems quite satisfying, why the MEMREAD is not close at all?
  2. What other registers in the core could I look at in order to fasten the reads?
  3. Are there different bus width between reads and writes in the A35 Cluster?
  4. Is there any register accessible to software to configure the SCU (Snoop Control Unit, responsible of the arbitration of the cores accesses) to give more bandwidth to one specific core?

Thanks,

Gael

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