I use @stage to arrange multiple registers in one stage, but when I arrange more than 4 registers in one stage, I receive a warning, and then the extra registers are arranged into other stages.
However, when I observe through p4i, I find that the SRAM Blocks occupied by each register are actually very small, and there are still many SRAM Blocks unused.
How can I arrange more than 4 registers in one stage?
Someone like Vlad could give a more authoritative answer than I can, but if I recall correctly, Tofino hardware may have a designed-in limitation of at most 4 registers in one stage, and that limit is due to other parts of the hardware than how much SRAM those registers use.
Dear @LongP4,
@andyfingerhut gave you the right answer.
In your case, the critical resource are the so-called “Meter ALUs”, which are used to implement all stateful externs except counters (that is meters, registers, low-pass filters (LPFs), weighted random early discard externs (WREDs) and action selector externs). I know, that name migth be a little confusing, but that’s what it is.
The column “S/M ALUs”, located right between the MAP RAMs and the right-hand-side (6x8) group of SRAM blocks represents both the Meter ALUs (M) and the Statistic ALUs (S) – the latter are responsible for the counters. The very top square there is the Meter ALU, the one underneath is a Counter ALU, the one down is the next Meter ALU and so on.
Since this column contains 8 squares, that means that each stage contains 4 meter and 4 counter ALUs. Thus, you can have no more than 4 registers/meters/LPfs/WREDs/ActionSelectors (total) plus 4 counters in any stage.
P4i tool also has a useful “Resource Utilization” section that shows all the resources and their utilization per stage. If some resource is 100% utilized, (in your case you will see that in the stage in question Meter ALU utilization is 100%).
Happy hacking,
Vladimir
