Comparing Canister Cycles vs Performance Counter

The example walks through a scenario of charging cycles that includes inter-canister calls and how the performance counter relates to the work done by canisters.

Setup

1. Canisters A and B are on the same subnet.
2. The User sends an 1KiB Ingress Message to Canister A
3. The Canister A calls Canister B with an 2KiB Update Call
4. The Canister B responds to Canister A with an 300-byte reply
5. The Canister A responds to User with an 500-byte reply

Example Cycles Flow

Naming conventions:

• 1K == 1'000
• 1KiB == 1'024 bytes
• 1M == 1'000'000
• 1MiB == 1'048'576 bytes
• 1B == 1'000'000'000
• 1T == 1'000'000'000'000

Example Step	Canister A Cycles Balance	Canister A Perf Counter	Canister B Cycles Balance	Canister B Perf Counter
1. Initial state	1T	N/A1)	1T	N/A1)
2. Canister A receives 1KiB Ingress Message	-1,2M -1KiB*2K ≈999'996M	N/A1)	1T	N/A1)
3. Canister A starts execution 2)	-590K -5B/10*4 ≈997'996M	0 (5B limit)	1T	N/A1)
4. Canister A executes 100M WASM instructions	997'996M	+100M =100M (5B limit)	1T	N/A1)
5. Canister A sends 2KiB call to Canister B 3)	-260K -2KiB*1K -2MiB*1K -590K -5B/10*4 ≈993'897M	+3*217) +2KiB ≈100M (5B limit)	1T	N/A1)
6. Canister A finishes execution 4)	+(5B-100M)*4/10 ≈995'857M	N/A1)	1T	N/A1)
7. Canister B starts execution 2)	995'857M	N/A1)	-590K -5B/10*4 ≈997'999M	0 (5B limit)
8. Canister B executes 100M WASM instructions	995'857M	N/A1)	997'999M	+100M =100M (5B limit)
9. Canister B sends 300-byte reply to Canister A	995'857M	N/A1)	997'999M	+2*217) +300 ≈100M (5B limit)
10. Canister B finishes execution 4)	995'857M	N/A1)	+(5B-100M)*4/10 ≈999'959M	N/A1)
11. Canister A receives a reply from Canister B 5)	+(2MiB-300)*1K ≈997'953M	N/A1)	999'959M	N/A1)
12. Canister A starts reply callback execution 6)	997'953M	0 (5B limit)	999'959M	N/A1)
13. Canister A executes 100M WASM instructions	997'953M	+100M =100M (5B limit)	999'959M	N/A1)
14. Canister A sends 500-byte reply to User	997'953M	+2*21 +500 ≈100M (5B limit)	999'959M	N/A1)
15. Canister A finishes execution 4)	+(5B-100M)*4/10 ≈999'913M	N/A1)	999'959M	N/A1)
Total:	~87M Cycles for ~200M WASM Instructions Executed + messaging		~41M Cycles for ~100M WASM Instructions Executed + messaging

Notes:

1. The Performance Counter is available only during execution, so it's marked as N/A when the execution has not started yet.
2. The Canister get charged:

   590K

   for the update message execution

   5B/10*4

   for 5B instructions it can potentially execute

3. The Canister get charged:

   260K

   for the inter-canister call

   2KiB * 1K

   for byte transmission

   2MiB * 1K

   for the maximum reply size

   590K

   for the reply update message execution

   5B/10*4

   for 5B instructions limit it can potentially execute in the reply handler

4. The Canister gets a refund for the unused Instructions.
5. The Canister gets a refund for the unused reply bytes.
6. The Canister has already been charged in step 5 for the reply execution and 5B instructions.
7. The n*21 represents System API calls costs.

References

1. The Internet Computer Interface Specification
2. The Internet Computer Computation and Storage Costs
3. Community Conversations | Charging Canister Cycles