Filesystem accesses from pthreads are much slower than in the main thread. #3922
Description
In the following test case
#include <emscripten.h>
#include <malloc.h>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#define FILENAME "data.bin"
#define FILESIZE (1024*1024)
#define BLOCKSIZE (32*1024)
static void* read_whole_file(void* args)
{
FILE* file = (FILE*)args;
void* data = malloc(BLOCKSIZE);
double start = emscripten_get_now(), read_avrg = 0.0;
for( size_t i = 0; i < FILESIZE / BLOCKSIZE; ++i )
{
double read_start = emscripten_get_now();
size_t read = fread(data, 1, BLOCKSIZE, file);
double read_end = emscripten_get_now();
read_avrg += read_end - read_start;
if( read != BLOCKSIZE )
{
fprintf(stderr, "Error while reading file.\n");
exit(1);
}
}
double end = emscripten_get_now();
read_avrg /= FILESIZE / BLOCKSIZE;
free(data);
printf("Completed in %fms (~%fms per read of %u bytes)\n", end - start, read_avrg, BLOCKSIZE);
return args;
}
int main(int argc, char* argv[])
{
// First create random files for testing purposes
{
printf("Writing test file (%u bytes)...\n", FILESIZE);
FILE* file = fopen(FILENAME, "wb");
if( !file )
{
printf("Error while opening file for write.\n");
return 1;
}
const char data[] = {"abcdefghijklmnopqrstuvwxyz0123456789"};
for( size_t i = 0, c = FILESIZE / sizeof(data); i < c; ++i )
{
if( fwrite(data, 1, sizeof(data), file) != sizeof(data) )
{
printf("Error while writing to file.\n");
return 1;
}
}
size_t padding = FILESIZE % sizeof(data);
if( padding > 0 )
{
if( fwrite(data, 1, padding, file) != padding )
{
printf("Error while writing to file.\n");
return 1;
}
}
fclose(file);
}
// Test reading the file from main thread
{
printf("Reading file from main thread...\n");
FILE* file = fopen(FILENAME, "rb");
if( !file )
{
printf("Error while opening file for read.\n");
return 1;
}
read_whole_file(file);
fclose(file);
}
// Test reading the file from another thread
{
fprintf(stdout, "Reading file from another thread...\n");
FILE* file = fopen(FILENAME, "rb");
if( !file )
{
fprintf(stderr, "Error while opening file for read.\n");
return 1;
}
pthread_t thread;
pthread_create(&thread, NULL, &read_whole_file, file);
pthread_join(thread, NULL);
fclose(file);
}
printf("Done.\n");
return 0;
}
Compile with em++ a.cpp -o a.html -s USE_PTHREADS=1 -s PTHREAD_POOL_SIZE=1. File accesses from a thread are much slower than from the main thread, and the application prints something like
Preallocating 1 workers for a pthread spawn pool.
Writing test file (1048576 bytes)...
Reading file from main thread...
Completed in 11.740000ms (~0.362031ms per read of 32768 bytes)
Reading file from another thread...
Completed in 3232.535000ms (~101.005156ms per read of 32768 bytes)
Done.