Saturday, September 26, 2009

[TIP] XCode Header Search Path

For many of you, this sounds like a stupid thing. But for those that just use gcc -I from command line, can be a pain find how to do it.

So, the problem is. How can I specify my Include path in XCode (gcc -I./mypath).

Tap on your project target, and click "Get Info", tap on "Build" tab and search "Path" as showed in the figure above. Then click on the "Header Search Paths" options and add your favorite include paths for selected target.

Saturday, September 5, 2009

Grand Central Dispatch: First Look

In the last years I've always used a "parallel task" approach foreach loops that I've in the code, not always to speedup but even to clean-up the code. How to do it? Wrapping threads and Thread Pool like in this C# Parallel Forech Code.

Snow Leopard has introduced a new BSD-level infrastructure, with simple and efficient API to do this job. Here a little usage preview.

Block objects are a C-based language feature that you can use in your C, Objective-C, and C++ code. Blocks make it easy to define a self-contained unit of work. Blocks are something like Actions (delegate {}) in C#. Very useful to embed function in loops.

Blocks looks like a "private" function pointer, but you can access to the "parent" vars. (If you're a Python coder, you've exactly the same thing).

/* Blocks in Python...
* def main():
*    a = 10
*    def test(k):
*        print a, k
*    test(128)
int main (int argc, const char *argv[]) {
  int a = 12;

  void (^test_block) (int) = ^(int k) {
    printf("A Block: PARENT(%d) ARG(%d)\n", a, k);



The GCD queue API provides dispatch queues from which threads take tasks to be executed. Because the threads are managed by GCD, Mac OS X can optimize the number of threads based upon available memory, number of currently active CPU cores, and so on. This shifts a great deal of the burden of power and resource management to the operating system itself, freeing your application to focus on the actual work to be accomplished.

#include <dispatch/dispatch.h>
#include <stdlib.h>
#include <stdio.h>

#define ITEM_VMIN       (1)
#define ITEM_VMAX       (200)
#define NR_ITEMS        (100)

static void __fill_item (void *items, size_t n) {
  int *i_items = (int *)items;
  i_items[n] = (ITEM_VMIN + (int)(ITEM_VMAX * ((double)rand() / RAND_MAX)));

static void __work_on_item (void *items, size_t n) {
  int *i_items = (int *)items;
  i_items[n] *= 100;  /* Do some Computation on this Item */

int main (int argc, const char *argv[]) {
  dispatch_queue_t queue;
  int data[NR_ITEMS];

  /* Get Global Dispatch Queue */
  queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

  /* Initialize data Elements, and run computation on each element */
  dispatch_apply_f(NR_ITEMS, queue, data, __fill_item);
  dispatch_apply_f(NR_ITEMS, queue, data, __work_on_item);

  /* Brief review of the items */
  dispatch_apply(NR_ITEMS, queue, ^(size_t n) {
    printf("Results: Item %lu = %d\n", n, data[n]);


File-System and Data Block Back Reference

While I'm thinking and waiting for suggestions on how to improve my file-system block cache algorithm, I've decided to apply some changes to the Raleigh File-System Format (source code is not published yet).

Following the ideas of Valerie Aurora of Repair-driven File System Design, I've decided to add for each block (B*Tree and Data blocks) an head that contains a Magic Number and a CRC Sum of the block. In this way you can easily identify what kind of block you've peeked without scanning all metadata. Another step is to add a back reference (or back pointer) to the data block, in this way you can easily jump back to it's the extent block (and obviously to its OID) so you can easily understand what is the Object owner of this block and you can easily swap two blocks reading at most 4 blocks (2 Data and 2 Extends).

Another idea stolen from Valerie is to double the metadata blocks with a COW-like approach, as explained in this paper "Double the Metadata, Double the Fun: A COW-like Approach to FS Consistency", really useful for personal file-systems but maybe less in a distributed file-system. I'm working on it adding only as an mkfs option.

When the source Code will be online? I don't know.. I've less time to work on it. Maybe at the end of this year I'll publish the File-System and the Distributed System (explained some posts ago).
Double the Metadata, Double the Fun: A COW-like Approach to File

System Consistency"