hardware//I/O-bound
A workload is I/O-bound when waiting for input and output limits its speed: the processor sits idle while a network reply, a disk read, a database query or a remote model call is in transit. It is the third diagnosis beside compute-bound (arithmetic is the limit) and memory-bound (moving data between memory and the processor is the limit), and it decides which fix can work.
A workload is I/O-bound when waiting for input and output limits its speed: the processor sits idle while a network reply, a disk read, a database query or a remote model call is in transit. It is the third diagnosis beside compute-bound (arithmetic is the limit) and memory-bound (moving data between memory and the processor is the limit), and it decides which fix can work.
The test is where the time goes. A web scraper that fetches a page in 300 ms and parses it in 3 ms spends 99 % of its life waiting; a faster CPU would change nothing. An agent loop that calls a hosted LLM is the same: the model computes in a distant data center, and on the agent's machine the call is pure waiting, plus latency of the network both ways.
An I/O-bound program speeds up by waiting on many things at once, never by computing faster.
Overlap the waits with asynchronous programming or threads, cut their number (batching, caching), or move closer to what is being waited on.
The fixes differ from the compute-bound ones. Concurrency multiplies throughput almost for free (a hundred concurrent requests take about the time of one), until the remote side, a rate limit or the connection pool becomes the new wall.
The label depends on the setup. Parsing a million local files is compute-bound on a slow CPU and I/O-bound on a network drive; a PLC gateway polling fifty Modbus devices one at a time is bounded by the serial line, not by its processor.
Measurements beat intuition. A process at low CPU use with requests queueing is the signature; a profiler that shows time in recv, read or await confirms it.
The opposite cases, and how to tell them apart with arithmetic intensity, are in compute-bound and the roofline model.