linux-kernel - Size of 2.6.20 task_struct on x86

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Message-ID: <45CB4C55.4070902@redhat.com>
Date:	Thu, 08 Feb 2007 11:14:13 -0500
From:	William Cohen <wcohen@...hat.com>
To:	linux-kernel@...r.kernel.org
Subject: Size of 2.6.20 task_struct on x86_64 machines

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

View attachment "task_struct_compress.diff" of type "text/x-patch" (1364 bytes)