buildzoid

Why raising tREFI or lowering tRFC has diminshing returns on performance

Added 2023-11-20 16:49:02 +0000 UTC

This recently came up on the AHOC discord but I'm pretty sure I've mentioned this in a video or something but anyway:

As tRFC ÷ tREFI approaches 0 further reductions in tRFC or increases in tREFI will impact performance less and less.

tREFI increasing:
800tRFC with 8 000 tREFI = 10% of clock cycles are spent on refresh
800tRFC with 16 000 tREFI = 5% of clock cycles are spent on refresh
800tRFC with 32 000 tREFI = 2.5% of clock cycles are spent on refresh
800tRFC with 64 000 tREFI = 1.25% of clock cycles are spent on refresh
800 tRFC with 128 000 tREFI = 0.625% of clock cycles are spent on refresh

so going from 8K tREFI to 32K tREFI gives 7.5% more useful clock cycles >> big performance increase
going from 64K to 128K gives you an extra 0.625% >> basically no performance increase

tRFC decreasing:
800tRFC with 8 000 tREFI = 10% of clock cycles are spent on refresh
400tRFC with 8 000 tREFI = 5% of clock cycles are spent on refresh
200tRFC with 8 000 tREFI = 2.5% of clock cycles are spent on refresh
100tRFC with 8 000 tREFI = 1.25% of clock cycles are spent on refresh

As I hope you can see this leads to the same diminshing returns that raising tREFI has

This is also why I don't really bother with minimizing tRFC on daily memory OCs because once tRFC ÷ tREFI is close to 1% pushing either timing further isn't really gonna do much for performance but can harm stability.

incase you don't know what tRFC and tREFI are:

DDR memory needs to be "refreshed" in order to retain data because it stores data as charge on capacitors that slowly discharge over time.

tRFC is how long the memory controller waits after sending a REFRESH command before sending other commands (like ACTIVATE, READ, WRITE, PRECHARGE....)

tREFI is how often the memory controller sends REFRESH commands.

the timing values used here are just theoretically examples. What timings actually work for your RAM will depend on what RAM you have. Some memory sticks are incapable of running tRFCs of less than 600 at speeds higher than 1.8GHz (3600Mbps / MT/s)