DDR performance drop with specific address access pattern

Question

Hi, 
we are seeing performance drop when we access specific address pattern from DDR. Please find the below details.

in a single test we read same address repeatedly as mentioned below

rd_addr is 64B aligned and each read transaction is rd_len=1 (128B access)

'P' is the expected performance number

rd_addr =
 
0   -> no drop (P)

2   ->  no drop (P)

4   ->  **drop (~P/4)**

6   ->  **drop (~P/4)**

8   ->  no drop (P)

10 ->  no drop (P)

12 ->  **drop (~P/4)**

14 ->  **drop (~P/4)**

..........this pattern repeats........

following  are the results if we read more than 1 address repeatedly

{0, 2} -> no drop (P)

{4, 6} -> **drop (~P/2)**

{2, 4} -> **drop (~P/2)**

{0, 2, 4, 6} -> no drop (P)

{8 ,10} -> no drop (P)

{4 ,  12} -> **drop (~P/2)**

{4, 6, 8, 10} -> no drop (P)

{4 ,6 ,12, 14} -> no drop (P)

is there any known DDR performance limitation for the above address pattern?

Thanks

Indra

awschakra · Answer

Hello,

The DDR Controllers in F1 are configured to maximize the performance for incremental/sequential address patterns using the "**ROW_COLUMN_BANK_INTLV**"  addressing mode and "**Force Read and Write commands to use AutoPrecharge**" option enabled in the IP. When these options are selected, the Memory Controller issues a transaction to memory with an AutoPrecharge if Column address bit A3 is set High. Please see [PG150](https://docs.xilinx.com/v/u/en-US/pg150-ultrascale-memory-ip#page=202) for details on this configuration. Apparently, Column Address bit A3 corresponds to AXI4_Address bit[8].

If the user access patterns are to the same addresses (non-incremental pattern) then it is possible that the controller auto closes the current Bank/Row after a transaction but incurs penalty of re-opening the same bank/row and hence resulting in poor performance.

Please let us know if you have any questions.

Thanks!
Chakra

DDR performance drop with specific address access pattern

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