[PATCH] latencytop: note task_struct bloat

[PATCH] latencytop: note task_struct bloat

[Hugh Dickins]

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@tiscali.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

Re: [PATCH] latencytop: note task_struct bloat

[Ingo Molnar]

* Hugh Dickins <hugh.dickins@tiscali.co.uk> wrote:

> 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@tiscali.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.
> +

Ah, nasty - quadratics in action.

This should be improved really. Arjan, what would it take to convert 
latencytop over to perfcounters? What would be ideal software 
counter for this?

	Ingo