This past week I was playing around with that pahole tool
(http://oops.ghostprotocols.net:81/acme/dwarves/) and looking at the
size of various struct in the kernel. I was surprised by the size of
the task_struct on x86_64, approaching 4K. I looked through the
fields in task_struct and found that a number of them were declared as
"unsigned long" rather than "unsigned int" despite them appearing okay
as 32-bit sized fields. On x86_64 "unsigned long" ends up being 8
bytes in size and forces 8 byte alignment. Is there a reason there
a reason they are "unsigned long"?
The patch below drops the size of the struct from 3808 bytes (60
64-byte cachelines) to 3760 bytes (59 64-byte cachelines). A couple
other fields in the task struct take a signficant amount of space:
struct thread_struct thread; 688
struct held_lock held_locks[30]; 1680
CONFIG_LOCKDEP is turned on in the .config
-Will
--- include/linux/sched.h.compress 2007-02-06 16:16:14.000000000 -0500
+++ include/linux/sched.h 2007-02-07 18:09:34.000000000 -0500
@@ -802,8 +802,8 @@
volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */
struct thread_info *thread_info;
atomic_t usage;
- unsigned long flags; /* per process flags, defined below */
- unsigned long ptrace;
+ unsigned int flags; /* per process flags, defined below */
+ unsigned int ptrace;
int lock_depth; /* BKL lock depth */
@@ -826,7 +826,7 @@
unsigned long long sched_time; /* sched_clock time spent running */
enum sleep_type sleep_type;
- unsigned long policy;
+ unsigned int policy;
cpumask_t cpus_allowed;
unsigned int time_slice, first_time_slice;
@@ -846,11 +846,11 @@
/* task state */
struct linux_binfmt *binfmt;
- long exit_state;
+ int exit_state;
int exit_code, exit_signal;
int pdeath_signal; /* The signal sent when the parent dies */
/* ??? */
- unsigned long personality;
+ unsigned int personality;
unsigned did_exec:1;
pid_t pid;
pid_t tgid;
@@ -882,7 +882,7 @@
int __user *set_child_tid; /* CLONE_CHILD_SETTID */
int __user *clear_child_tid; /* CLONE_CHILD_CLEARTID */
- unsigned long rt_priority;
+ unsigned int rt_priority;
cputime_t utime, stime;
unsigned long nvcsw, nivcsw; /* context switch counts */
struct timespec start_time;
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