Throughput vs latency in computer architecture

Question

I've come across articles on "through-put vs latency" in contexts like networking e.g. https://homepage.cs.uri.edu/~thenry/resources/unix_art/ch12s04.html But in the context of computer architecture / operating systems, I'm not able to understand why would there be a trade-off between latency (response time of a program) and through-put (how many programs we're able to complete in a unit of time, say per hour). Is this solely due to the fact that we can choose to parallelize processing of multiple programs / requests leading to overheads like context switches & sharing of caches which make the start-to-end response time per process to be worse? Or am I missing something here?

Answer 1

In terms of single instructions in a superscalar pipelined out-of-order exec CPU, throughput vs. latency is very important because the CPU is trying to extract parallelism from an instruction stream that has to be executed as if in serial program order. See Assembly - How to score a CPU instruction by latency and throughput and the bottom of my answer on latency vs throughput in intel intrinsics for example.

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In terms of OS decisions that affect throughput vs. latency on a much longer timescale than a few clock cycles, that's a totally separate question.

One of the major factors there is choosing how to use the available physical RAM, and whether to page out (to a swap file) infrequently used code / data to make more room to cache disk files. (e.g. Linux's vm.swappiness is widely considered a key tunable in terms of setting it differently between servers and desktops. https://unix.stackexchange.com/questions/88693/why-is-swappiness-set-to-60-by-default).

If you alt-tab to a window when many pages of that process have been paged out, it will take some time before the process can redraw its window. (Multiple hard page faults, can be quite slow especially if paging on a rotational disk, not SSD.) So to optimize for latency, you want the kernel to not aggressively swap out pages from running processes, even if they've been idle for a few hours. Those pages, if they'd been free, could have improved throughput for other processes by acting as buffers / cache.

A related factor is I/O scheduling: trying to group IO requests together to minimize HD seek times (for higher throughput and lower average latency), but sometimes at the expense of delaying a few requests for a longer time (higher worst-case latency). Linux for example has many to choose from, including deadline, Completely Fair Queuing (CFQ), and the original elevator (just grouping requests by locality without consideration of fairness or latency). https://wiki.archlinux.org/title/improving_performance#Input/output_schedulers

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CPU scheduling is also a factor: a context-switch hurts throughput, as it takes time itself and caches will likely be cold for the new task on this CPU. You also have to run the kernel's schedule() function to decide which task to run next, so that takes away some time from real work.

To minimize latency (for example between a socket message being sent to a process and it waking up when its poll or select system call returns), you want a short timeslice, like Linux HZ=1000. (Timer interrupts every 1 ms to run the scheduler). And you want to be able to pre-empt even the kernel itself, instead of waiting until the kernel is ready to return to the old user-space to consider the possibility of running a different user-space task.

But neither of these helps throughput, and in fact hurt (assuming the workload has enough parallelism to not bottleneck on latency). So HZ=100 was the default for "server" Linux builds, vs. 1000 on "desktop" builds tuned for interactive use. (Modern Linux can be "tickless", not using a fixed timer interrupt on every core at all, instead deciding when to schedule the next interrupt on a case by case basis.)

Real-time kernels take this even further, spending more time on finer-grained locking and stuff like that to enable pausing work and coming back to it later to minimize interrupt latency and other latencies between it being time to do something and actually starting to do that thing. (There are real-time patches for Linux, and there are also totally separate kernels built from the ground up for real-time operation.)

If you have an embedded system controlling a motor or something, you absolutely need hard real-time latency guarantees that it will never take longer than say 1 millisecond from an interrupt pin being asserted to the interrupt handler starting to run.

(Designing the system to make these guarantees possible often comes at the cost of throughput. e.g. obviously you have to pin some memory to make it not swappable, if we're talking about user-space, making it unavailable for cache even if it goes untouched for days.)