linux-kernel - [PATCH] latencytop: note task

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Message-ID: <Pine.LNX.4.64.0906081841580.7100@sister.anvils>
Date:	Mon, 8 Jun 2009 18:44:48 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Arjan van de Ven <arjan@...radead.org>
cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: [PATCH] latencytop: note task_struct bloat

Add belated comment to LATENCYTOP help text, making clear that this is a
much more expensive option than you might think: along with the options
it selects, it may add more than 4000 bytes to each struct task_struct
(on 64-bit; more than 2000 on 32-bit), effectively quadrupling the size
of an otherwise lean task_struct (in fact, slab on x86_64 fits only 1
in two pages, whereas it fits 5 in two pages without LATENCYTOP=y).

Signed-off-by: Hugh Dickins <hugh.dickins@...cali.co.uk>
---

 lib/Kconfig.debug |    4 ++++
 1 file changed, 4 insertions(+)

--- 2.6.30-rc8/lib/Kconfig.debug	2009-05-16 10:26:16.000000000 +0100
+++ linux/lib/Kconfig.debug	2009-06-08 18:30:50.000000000 +0100
@@ -826,6 +826,10 @@ config LATENCYTOP
 	  Enable this option if you want to use the LatencyTOP tool
 	  to find out which userspace is blocking on what kernel operations.
 
+	  Note: enabling this option may add more than 4000 bytes to each
+	  task_struct on a 64-bit kernel (more than 2000 bytes to task_struct
+	  on a 32-bit kernel), multiplying its memory usage by 4 or even 5.
+
 config SYSCTL_SYSCALL_CHECK
 	bool "Sysctl checks"
 	depends on SYSCTL_SYSCALL
--
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