Thread-safety of read-only memory access

Posted by Edmund on Stack Overflow See other posts from Stack Overflow or by Edmund
Published on 2010-05-04T04:22:26Z Indexed on 2010/05/04 4:28 UTC
Read the original article Hit count: 234

Filed under:

thread-safety

I've implemented the Barnes-Hut gravity algorithm in C as follows:

Build a tree of clustered stars.
For each star, traverse the tree and apply the gravitational forces from each applicable node.
Update the star velocities and positions.

Stage 2 is the most expensive stage, and so is implemented in parallel by dividing the set of stars. E.g. with 1000 stars and 2 threads, I have one thread processing the first 500 stars and the second thread processing the second 500.

In practice this works: it speeds the computation by about 30% with two threads on a two-core machine, compared to the non-threaded version. Additionally, it yields the same numerical results as the original non-threaded version.

My concern is that the two threads are accessing the same resource (namely, the tree) simultaneously. I have not added any synchronisation to the thread workers, so it's likely they will attempt to read from the same location at some point. Although access to the tree is strictly read-only I am not 100% sure it's safe. It has worked when I've tested it but I know this is no guarantee of correctness!

Questions

Do I need to make a private copy of the tree for each thread?
Even if it is safe, are there performance problems of accessing the same memory from multiple threads?

Related posts about pthreads

gcc: Do I need -D_REENTRANT with pthreads?

as seen on Stack Overflow - Search for 'Stack Overflow'
On Linux (kernel 2.6.5) our build system calls gcc with -D_REENTRANT. Is this still required when using pthreads? How is it related to gcc -pthread option? I understand that I should use -pthread with pthreads, do I still need -D_REENTRANT? On a side note, is there any difference that you know… >>> More
pthreads: reader/writer locks, upgrading read lock to write lock

as seen on Stack Overflow - Search for 'Stack Overflow'
I'm using read/write locks on Linux and I've found that trying to upgrade a read locked object to a write lock deadlocks. i.e. // acquire the read lock in thread 1. pthread_rwlock_rdlock( &lock ); // make a decision to upgrade the lock in threads 1. pthread_rwlock_wrlock( &lock ); // this… >>> More
SUA + Visual Studio + pthreads

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi, I cannot compile this code under SUA: #include <unistd.h> #include <stdio.h> #include <stdlib.h> #include <pthread.h> void * thread_function(void *arg) { printf("thread_function started. Arg was %s\n", (char *)arg); // pause for 3 seconds sleep(3); … >>> More
Creating an independent draw thread using pthreads (C++)

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi all, I'm working on a graphical application which looks something like this: while (Simulator.simulating) { Simulator.update(); InputManager.processInput(); VideoManager.draw(); } I do this several times a second, and in the vast majority of cases my computation will be taking up… >>> More
posix pthreads in c

as seen on Stack Overflow - Search for 'Stack Overflow'
Iam new to c programming and needs some help. long *taskids[NUM_THREADS]; for(t=0; t<NUM_THREADS; t++) { taskids[t] = (long *) malloc(sizeof(long)); *taskids[t] = t; printf("Creating thread %ld\n", t); rc = pthread_create(&threads[t], NULL, PrintHello, (void *) taskids[t]); ... } This… >>> More

Developer IT