Asynchronous and more versatile replacement of posix_spawn
aspawn implements the idea from avfork, enabling user to do posix_spawn in an asynchronous way by
making syscall directly via pure_syscall implemented in my library that does not use
any global/thread local variable at all.
My aspawn has signature:
struct Stack_t {
void *addr;
size_t size;
};
typedef int (*aspawn_fn)(void *arg, int wirte_end_fd, void *old_sigset, void *user_data, size_t user_data_len);
/**
* @return fd of read end of CLOEXEC pipe if success, eitherwise (-errno).
*
* aspawn would disable thread cancellation, then it would revert it before return.
*
* aspawn would also mask all signals in parent and reset the signal handler in the child process.
* Before aspawn returns in parent, it would revert the signal mask.
*
* In the function fn, you can only use syscall declared in syscall/syscall.h
* Use of any glibc function or any function that modifies global/thread-local variable is undefined behavior.
*/
int aspawn(pid_t *pid, struct Stack_t *cached_stack, size_t reserved_stack_sz,
aspawn_fn fn, void *arg, void *user_data, size_t user_data_len);
By returning the write end of the CLOEXEC pipefd, user of this library is able to receive error message/check whether
the child process has done using cached_stack so that aspawn can reuse cached_stack.
It also allows user to pass arbitary data in the stack via user_data and user_data_len, which get copies onto top of
the stack, thus user does not have to allocate them separately on heap or mistakenly overwriten an object used in child process.
To use a syscall, you need to include syscall/syscall.h, which defines the syscall routine used by the child process including
find_exe, psys_execve and psys_execveat.
Compare to posix_spawn, aspawn has 3 advantages:
aspawnallows user to do anything in the child process beforeexec.aspawncan reuse stack,posix_spawncan't;aspawndoesn't block the parent thread;
Responsive comparison between posix_spawn and aspawn, source code:
$ ll -h bench_aspawn_responsiveness.out
-rwxrwxr-x 1 nobodyxu nobodyxu 254K Oct 2 15:02 bench_aspawn_responsiveness.out*
$ uname -a
Linux pop-os 5.4.0-7642-generic #46~1598628707~20.04~040157c-Ubuntu SMP Fri Aug 28 18:02:16 UTC x86_64 x86_64 x86_64 GNU/Linux
$ ./a.out
2020-10-02T15:02:45+10:00
Running ./bench_aspawn_responsiveness.out
Run on (12 X 4100 MHz CPU s)
CPU Caches:
L1 Data 32 KiB (x6)
L1 Instruction 32 KiB (x6)
L2 Unified 256 KiB (x6)
L3 Unified 9216 KiB (x1)
Load Average: 0.31, 0.36, 0.32
---------------------------------------------------------------------
Benchmark Time CPU Iterations
---------------------------------------------------------------------
BM_aspawn_no_reuse 18009 ns 17942 ns 38943
BM_aspawn/threads:1 14500 ns 14446 ns 48339
BM_vfork_with_shared_stack 46545 ns 16554 ns 44027
BM_fork 54583 ns 54527 ns 12810
BM_posix_spawn 125061 ns 29091 ns 24483
Since aspawn allows user to do anything in the vforked child via aspawn_fn, it makes no sense
to benchmark how many processes can aspawn created as it depends on user provided argument fn.
Make sure that you have installed make, clang lld and llvm-ar.
Then run make -j $(nproc) to build the project, sudo make install to install project to /usr/local/.
Make sure you have installed all depedencies listed above for building this project,
then run make test -j $(nproc)
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