March 2009 Archives

Unlike my default shell (zsh), bash has a wonderful feature where it doesn't keep variables that are set at the other end of a pipe, so for example:
i=
cat foo | while read bar; do
    i=$bar
done
echo $i
Yields an empty line. I've been stung once or twice on this as I prototype the code initially in an interactive shell, which doesn't exhibit the issue.
The simplest solution is to use a named pipe.

i=
mkfifo /tmp/foo$$
cat foo >/tmp/foo$$&
pid=$!
while read bar; do
    i=$bar
done </tmp/foo$$
This gives the last line of the file in the i variable.

So there I am looking at the sysinfo from a particular machine and I look at the content of the procs field. It looked to be a bit out. Went hunting through the kernel source and noticed that the procs field is filled with the value of the number of threads in the system. This is a little bit odd, as I'm used to separating my threads from my processes.
Turns out that there is an nr_processes() call, which returns the number of processes in the system, rather than the number of threads. A little bit of a change, rebuild and testing now gives me the correct number of processes from the proc field, and I also have a separate result for the number of threads.
There we go, much more sensible ;)
diff -Naur linux-2.6.25.18/include/linux/kernel.h linux-2.6.25.18.new/include/linux/kernel.h
--- linux-2.6.25.18/include/linux/kernel.h      2008-10-09 03:58:32.000000000 +0100
+++ linux-2.6.25.18.new/include/linux/kernel.h  2009-03-16 16:23:39.000000000 +0000
@@ -415,7 +415,8 @@
        unsigned long totalhigh;        /* Total high memory size */
        unsigned long freehigh;         /* Available high memory size */
        unsigned int mem_unit;          /* Memory unit size in bytes */
-       char _f[20-2*sizeof(long)-sizeof(int)]; /* Padding: libc5 uses this.. */
+       unsigned int threads;           /* Number of current threads */
+       char _f[20-2*sizeof(long)-2*sizeof(int)];       /* Padding: libc5 uses this.. */
 };
 
 /* Force a compilation error if condition is true */
diff -Naur linux-2.6.25.18/kernel/compat.c linux-2.6.25.18.new/kernel/compat.c
--- linux-2.6.25.18/kernel/compat.c     2008-10-09 03:58:32.000000000 +0100
+++ linux-2.6.25.18.new/kernel/compat.c 2009-03-16 16:43:31.000000000 +0000
@@ -1031,7 +1031,8 @@
        u32 totalhigh;
        u32 freehigh;
        u32 mem_unit;
-       char _f[20-2*sizeof(u32)-sizeof(int)];
+       u32 threads;
+       char _f[20-2*sizeof(u32)-2*sizeof(int)];
 };
 
 asmlinkage long
@@ -1076,7 +1077,8 @@
            __put_user (s.procs, &info->procs) ||
            __put_user (s.totalhigh, &info->totalhigh) ||
            __put_user (s.freehigh, &info->freehigh) ||
-           __put_user (s.mem_unit, &info->mem_unit))
+           __put_user (s.mem_unit, &info->mem_unit) ||
+           __put_user (s.threads, &info->threads))
                return -EFAULT;
 
        return 0;
diff -Naur linux-2.6.25.18/kernel/timer.c linux-2.6.25.18.new/kernel/timer.c
--- linux-2.6.25.18/kernel/timer.c      2008-10-09 03:58:32.000000000 +0100
+++ linux-2.6.25.18.new/kernel/timer.c  2009-03-16 16:20:02.000000000 +0000
@@ -37,6 +37,7 @@
 #include <linux/delay.h>
 #include <linux/tick.h>
 #include <linux/kallsyms.h>
+#include <linux/sched.h>
 
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
@@ -1166,7 +1167,8 @@
                info->loads[1] = avenrun[1] << (SI_LOAD_SHIFT - FSHIFT);
                info->loads[2] = avenrun[2] << (SI_LOAD_SHIFT - FSHIFT);
 
-               info->procs = nr_threads;
+               info->procs = nr_processes();
+               info->threads = nr_threads;
        } while (read_seqretry(&xtime_lock, seq));
 
        si_meminfo(info);

The Full patch.

That's a huge amount of fragmentation. I think I may not be able to survive that percentage. It took < 5 seconds to defrag. Please people, percentages are supposed to mean something ;)