usched: stackswap coroutines, neither stackful nor stackless

[Please read the update.]

This repository (https://github.com/nikitadanilov/usched) contains a simple experimental implementation of coroutines alternative to well-known "stackless" and "stackful" methods.

The term "coroutine" gradually grew to mean a mechanism where a computation, which in this context means a chain of nested function calls, can "block" or "yield" so that the top-most caller can proceed and the computation can later be resumed at the blocking point with the chain of intermediate function activation frames preserved.

Prototypical uses of coroutines are lightweight support for potentially blocking operations (user interaction, IO, networking) and generators, which produce multiple values (see same fringe problem).

There are two common coroutine implementation methods:

  • a stackful coroutine runs on a separate stack. When a stackful coroutine blocks, it performs a usual context switch. Historically "coroutines" meant stackful coroutines. Stackful coroutines are basically little more than usual threads, and so they can be kernel (supported by the operating system) or user-space (implemented by a user-space library, also known as green threads), preemptive or cooperative.

  • a stackless coroutine does not use any stack when blocked. In a typical implementation instead of using a normal function activation frame on the stack, the coroutine uses a special activation frame allocated in the heap so that it can outlive its caller. Using heap-allocated frame to store all local variable lends itself naturally to compiler support, but some people are known to implement stackless coroutines manually via a combination of pre-processing, library and tricks much worse than Duff's device.

Stackful and stateless are by no means the only possibilities. One of the earliest languages to feature generators CLU (distribution) ran generators on the caller's stack.

usched is in some sense intermediate between stackful and stackless: its coroutines do not use stack when blocked, nor do they allocate individual activation frames in the heap.

The following is copied with some abbreviations from usched.c.


usched: A simple dispatcher for cooperative user-space threads.

A typical implementation of user-space threads allocates a separate stack for each thread when the thread is created and then dispatches threads (as decided by the scheduler) through some context switching mechanism, for example, longjmp().

In usched all threads (represented by struct ustack) are executed on the same "native" stack. When a thread is about to block (usched_block()), a memory buffer for the stack used by this thread is allocated and the stack is copied to the buffer. After that the part of the stack used by the blocking thread is discarded (by longjmp()-ing to the base of the stack) and a new thread is selected. The stack of the selected thread is restored from its buffer and the thread is resumed by longjmp()-ing to the usched_block() that blocked it.

The focus of this implementation is simplicity: the total size of usched.[ch] is less than 120LOC, as measured by SLOCCount.


  • no need to allocate maximal possible stack at thread initialisation: stack buffer is allocated as needed. It is also possible to free the buffer when the thread is resumed (not currently implemented);

  • thread that doesn't block has 0 overhead: it is executed as a native function call (through a function pointer) without any context switching;

  • because the threads are executed on the stack of the same native underlying thread, native synchronisation primitives (mutices, etc.) work, although the threads share underlying TLS. Of course one cannot use native primitives to synchronise between usched threads running on the same native thread.


  • stack copying introduces overhead (memcpy()) in each context switch;

  • because stacks are moved around, addresses on a thread stack are only valid while the thread is running. This invalidates certain common programming idioms: other threads and heap cannot store pointers to the stacks, at least to the stacks of the blocked threads. Note that Go language, and probably other run-times, maintains a similar invariant.


usched is only a dispatcher and not a scheduler: it blocks and resumes threads but

  • it does not keep track of threads (specifically allocation and freeing of struct ustack instances is done elsewhere),

  • it implements no scheduling policies.

These things are left to the user, together with stack buffers allocation and freeing. The user supplies 3 call-backs:

  • usched::s_next(): the scheduling function. This call-backs returns the next thread to execute. This can be either a new (never before executed) thread initialised with ustack_init(), or it can be a blocked thread. The user must keep track of blocked and runnable threads, presumably by providing wrappers to ustack_init() and ustack_block() that would record thread state changes. It is up to usched::s_next() to block and wait for events if there are no runnable threads and all threads are waiting for something;

  • usched::s_alloc(): allocates new stack buffer of at least the specified size. The user have full control over stack buffer allocation. It is possible to pre-allocate the buffer when the thread is initialised (reducing the cost of usched_block()), it is possible to cache buffers, etc.;

  • usched::s_free(): frees the previously allocated stack buffer.

rr.h and rr.c provide a simple "round-robin" scheduler implementing all the call-backs. Use it carefully, it was only tested with rmain.c benchmark.


The following diagrams show stack management by usched. The stack grows from right to left.

At the entrance to the dispatcher loop. usched_run(S):

| buf | anchor | ... |
sp = S->s_buf

A new (never before executed) thread U is selected by S->s_next(), launch() calls the thread startup function U->u_f():

U->u_f() launch() usched_run()
| | pad | buf | anchor | ... |
^ ^
| |
sp U->u_bottom

The thread executes as usual on the stack, until it blocks by calling usched_block():

usched_block() bar() U->u_f() launch() usched_run()
| here | ... | | | pad | buf | anchor | ... |
^ ^ ^
| +-- sp = U->u_cont |
| U->u_bottom

The stack from U->u_top to U->u_bottom is copied into the stack buffer U->u_stack, and control returns to usched_run() by longjmp(S->s_buf):

| buf | anchor | ... |
sp = S->s_buf

Next, suppose S->s_next() selects a previously blocked thread V ready to be resumed. usched_run() calls cont(V).

cont() usched_run()
| | buf | anchor | ... |

cont() copies the stack from the buffer to [V->u_top, V->u_bottom] range. It's important that this memcpy() operation does not overwrite cont()'s own stack frame, this is why pad[] array is needed in launch(): it advances V->u_bottom and gives cont() some space to operate.

usched_block() foo() V->u_f() cont() usched_run()
| here | ... | | | | | buf | anchor | ... |
^ ^ ^ ^
| +-- V->u_cont | +-- sp
| |
V->u_top V->u_bottom

Then cont() longjmp()-s to V->u_cont, restoring V execution context:

usched_block() foo() V->u_f() cont() usched_run()
| here | ... | | | | | buf | anchor | ... |
+-- sp = V->u_cont

V continues its execution as if it returned from usched_block().


By design, a single instance of struct usched cannot take advantage of multiple processors, because all its threads are executing within a single native thread. Multiple instances of struct usched can co-exist within a single process address space, but a ustack thread created for one instance cannot be migrated to another. One possible strategy to add support for multiple processors is to create multiple instances of struct usched and schedule them (that is, schedule the threads running respective usched_run()-s) to processors via pthread_setaffinity_np() or similar. See rr.c for a simplistic implementation.

Current limitations

  • the stack is assumed to grow toward lower addresses. This is easy to fix, if necessary;

  • the implementation is not signal-safe. Fixing this can be as easy as replacing *jmp() calls with their sig*jmp() counterparts. At the moment signal-based code, like gperf -lprofiler library, would most likely crash usched;

  • usched.c must be compiled without optimisations and with -fno-stack-protector option (gcc);

  • usched threads are cooperative: a thread will continue to run until it completes of blocks. Adding preemption (via signal-based timers) is relatively easy, the actual preemption decision will be relegated to the external "scheduler" via a new usched::s_preempt() call-back invoked from a signal handler.


Midori seems to use a similar method: a coroutine (called activity there) starts on the native stack. If it needs to block, frames are allocated in the heap (this requires compiler support) and filled in from the stack, the coroutine runs in these heap-allocated frames when resumed.


usched was benchmarked against a few stackful (go, pthreads) and stackless (rust, c++ coroutines) implementations. A couple of caveats:

  • all benchmarking in general is subject to the reservations voiced by Hippocrates and usually translated (with the complete reversal of the meaning) as ars longa, vita brevis, which means: "the art [of doctor or tester] takes long time to learn, but the life of a student is brief, symptoms are vague, chances of success are doubtful".

  • the author is much less than fluent with all the languages and frameworks used in the benchmarking. It is possible that some of the benchmarking code is inefficient or just outright wrong. Any comments are appreciated.

The benchmark tries to measure the efficiency of coroutine switching. It creates R cycles, N coroutines each. Each cycle performs M rounds, where each round consists of sending a message across the cycle: a particular coroutine (selected depending on the round number) sends the message to its right neighbour, all other coroutines relay the message received from the left to the right, the round completes when the originator receives the message after it passed through the entire cycle.

If N == 2, the benchmark is R pairs of processes, ping-ponging M messages within each pair.

Some benchmarks support two additional parameters: D (additional space in bytes, artificially consumed by each coroutine in its frame) and P (the number of native threads used to schedule the coroutines.

The benchmark creates N*R coroutines and sends a total of N*R*M messages, the latter being proportional to the number of coroutine switches.

bench.sh runs all implementations with the same N, R and M parameters. graph.py plots the results.

  • POSIX. Source: pmain.c, binary: pmain. Pthreads-based stackful implementation in C. Uses default thread attributes. pmain.c also contains emulation of unnamed POSIX semaphores for Darwin. Plot label: "P". This benchmarks crashes with "pmain: pthread_create: Resource temporarily unavailable" for large values of N*R.

  • Go. Source: gmain.go, binary: gmain. The code is straightforward (it was a pleasure to write). D is supported via runtime.GOMAXPROCS(). "GO1T" are the results for a single native thread, "GO" are the results without the restriction on the number of threads.

  • Rust. Source: cycle/src/main.rs, binary: cycle/target/release/cycle. Stackless implementation using Rust builtin async/.await. Label: "R". It is single-threaded (I haven't figured out how to distribute coroutines to multiple executors), so should be compared with GO1T, C++1T and U1T. Instead of fighting with the Rust borrow checker, I used "unsafe" and shared data-structures between multiple coroutines much like other benchmarks do.

  • C++. Source: c++main.cpp, binary: c++main. The current state of coroutine support in C++ is unclear. Is everybody supposed to directly use <coroutine> interfaces or one of the mutually incompatible libraries that provide easier to use interfaces on top of <coroutine>? This benchmark uses Lewis Baker's cppcoro, (Andreas Buhr's fork). Labels: "C++" and "C++1T" (for single-threaded results).

  • usched. Source: rmain.c, binary: rmain. Based on usched.[ch] and rr.[ch] This is our main interest, so we test a few combinations of parameters.

    • Label: "U": the default configuration, round-robin scheduler over 16 native threads,
    • "U1K": 1000 bytes of additional stack space for each coroutine
    • "U10K": 10000 bytes,
    • "U1T": round-robin over 1 native thread,
    • "U1TS": round-robin over 1 native thread with pthread locking in rr.c compiled out (-DSINGLE_THREAD compilation option, a separate binary rmain.1t).
    • Update "UL": uses "local" scheduler ll.c. All coroutines within a cycle are assigned to the same native thread so that scheduling between them require no locking. This demonstrates very high throughput (comparable to C++), but unfortunately I do not have time right now to re-do all the measurements consistently. Binary: lmain.

bench.sh runs all benchmarks with N == 2 (message ping-pong) and N == 8. Raw results are in results.linux. In the graphs, the horizontal axis is the number of coroutines (N*R, logarithmic) and the vertical axis is the operations (N*R*M) per second

Environment: Linux VM, 16 processors, 16GB of memory. Kernel: 4.18.0 (Rocky Linux).

16 32 64 400 800 4000 8000 40000 80000 400000 800000 4000000 8000000
POSIX 1.76 3.46 6.39 14.58 14.85 14.70 13.63 9.87 8.02 0.00 0.00 0.00 0.01
GO 4.14 5.62 7.77 36.74 41.64 49.72 48.24 37.24 43.06 46.31 46.22 46.09 45.95
GO1T 4.38 4.30 4.27 4.11 3.81 3.53 3.40 3.33 3.43 3.99 3.98 3.95 3.86
RUST 9.48 8.71 8.69 8.64 8.53 7.85 6.59 4.32 3.80 3.63 3.63 3.83 3.90
U 17.24 17.27 17.30 25.77 29.99 71.68 77.32 78.92 77.98 80.88 82.09 83.66 82.15
U1K 16.21 16.29 16.35 25.38 28.41 69.92 75.76 74.31 73.65 76.69 76.75 75.84 76.56
U10K 9.04 8.96 9.09 20.38 21.69 58.13 60.95 59.66 60.50 61.32 61.71 62.06 62.72
U1T 17.37 17.31 17.35 17.35 17.36 17.27 17.29 17.14 17.06 16.91 16.91 16.91 16.87
C++ 49.87 67.85 74.94 73.91 73.04 62.48 59.15 57.23 56.48 55.35 55.44 54.02 53.61
C++1T 97.03 97.38 96.82 96.06 96.58 95.78 94.83 89.83 86.09 80.48 79.37 77.04 77.48
U1TS 49.53 49.76 49.83 50.16 49.93 48.88 49.75 48.75 47.99 46.48 46.25 45.99 46.12
UL 76.03 116.63 160.72 169.74 169.99 171.57 170.32 165.82 169.43 174.32 171.55 169.48 170.04

(N == 8) A few notes:

  • As mentioned above, pthreads-based solution crashes with around 50K threads.

  • Most single-threaded versions ("GO1T", "R" and "U1T") are stable as corpse's body temperature. Rust cools off completely at about 500K coroutines. Single-threaded C++ ("C++1T") on the other hand is the most performant solution for almost the entire range of measurement, it is only for coroutine counts higher than 500K when "U" overtakes it.

  • It is interesting that a very simple and unoptimised usched fares so well against heavily optimized C++ and Go run-times. (Again, see the reservations about the benchmarking.)

  • Rust is disappointing: one would hope to get better results from a rich type combined with compiler support.

4 8 16 100 200 1000 2000 10000 20000 100000 200000 1000000 2000000
POSIX 0.56 0.97 1.84 6.36 6.88 7.78 7.82 7.58 7.15 5.34 0.00 0.00 0.00
GO 7.40 11.03 19.23 40.44 45.79 51.81 52.87 52.77 53.15 53.62 53.22 55.77 56.82
GO1T 4.54 4.55 4.53 4.53 4.53 4.52 4.52 4.50 4.47 4.36 4.31 4.26 4.26
RUST 5.68 5.75 5.75 4.74 4.74 4.62 4.46 4.13 3.70 2.81 2.77 2.76 2.73
U 11.22 11.27 11.26 11.30 7.91 24.66 38.72 35.67 40.60 41.18 42.06 42.96 42.74
U1K 9.64 9.62 9.65 9.67 7.61 22.14 34.38 31.70 34.54 34.56 34.59 35.47 35.56
U10K 4.43 4.62 4.50 4.25 5.02 15.79 26.18 25.33 27.60 27.62 27.63 27.72 28.16
U1T 11.24 11.29 11.34 11.26 11.32 11.30 11.28 11.28 11.22 11.19 11.15 11.13 11.15
C++ 46.33 46.30 63.38 114.30 117.05 114.12 111.36 101.32 100.13 84.30 78.53 72.77 71.00
C++1T 96.56 96.03 96.37 95.97 95.49 95.68 94.94 92.95 91.23 83.55 80.33 77.22 76.22
U1TS 19.59 19.66 19.80 19.87 19.89 19.86 19.82 19.72 19.66 19.51 19.45 19.33 19.37
UL 12.19 23.48 50.39 65.71 67.22 69.17 70.01 70.09 69.36 69.28 69.43 68.83 68.00

(N == 2)

  • First, note that the scale is different on the vertical axis.

  • Single-threaded benchmarks display roughly the same behaviour (exactly the same in "C++1T" case) as with N == 8.

  • Go is somewhat better. Perhaps its scheduler is optimised for message ping-pong usual in channel-based concurrency models?

  • usched variants are much worse (50% worse for "U") than N == 8.

  • Rust is disappointing.

To reproduce:

$ # install libraries and everything, then...
$ make
$ while : ;do ./bench.sh | tee -a results; sleep 5 ;done # collect enough results, this might take long...
$ grep -h '^ *[2N],' results | python3 graph.py c2.svg > c2-table.md # create plot for N == 2
$ grep -h '^ *[8N],' results | python3 graph.py c8.svg > c8-table.md # create plot for N == 8


Overall, the results are surprisingly good. The difference between "U1T" and "U1TS" indicates that the locking in rr.c affects performance significantly, and affects it even more with multiple native threads, when locks are contended across processors. I'll try to produce a more efficient (perhaps lockless) version of a scheduler as the next step.

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