CONFIG_FORCE_MAX_ZONEORDER default value

CONFIG_FORCE_MAX_ZONEORDER default value

[John Marvin]

I was wondering why CONFIG_FORCE_MAX_ZONEORDER was changed from 19 (in 2.4)
to 18 (in 2.6). Note that this value sets the maximum number of orders,
i.e. the actual maximum order is one less, i.e. 17. For the 16Kb default
page size this gives you a maximum page size of 2Gb, which isn't
particularly useful on ia64 since that size is not supported. The only
reason I can see 18 being useful would be to allow for 1Gb huge pages
on a system where PAGE_SIZE is set to 8Kb.

Anyway, with the default page size being 16K, it seems that it would
be more appropriate for CONFIG_FORCE_MAX_ZONEORDER to be either 17 (for
a 1 Gb max huge page size) or 19 (to support the maximum 4 Gb page size
available on Itanium 2 systems).

Is there another consideration that I've missed?

John Marvin

Re: CONFIG_FORCE_MAX_ZONEORDER default value

[Xavier Bru]

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Hi John & all

We also have a concern about CONFIG_FORCE_MAX_ZONEORDER=18 default value
with 64k page size, we get the message:
"BUG: wrong zone alignment, it will crash"
This is due to the normal zone (4 GB) not being aligned on the maximum 
order boundary (8GB).

A possible fix for this problem is to set the value to 17 for 64k pages...

linux-2.6.11-kgdb/arch/ia64/Kconfig
--- linux-2.6.11r/arch/ia64/Kconfig 2005-05-11 15:23:34.000000000 +0200
+++ linux-2.6.11/arch/ia64/Kconfig 2005-05-13 17:55:56.072557343 +0200
@@ -229,6 +229,7 @@ config IA64_SGI_SN_SIM

config FORCE_MAX_ZONEORDER
int
+ default "17" if IA64_PAGE_SIZE_64KB
default "18"

config SMP

John Marvin wrote:

>I was wondering why CONFIG_FORCE_MAX_ZONEORDER was changed from 19 (in 2.4)
>to 18 (in 2.6). Note that this value sets the maximum number of orders,
>i.e. the actual maximum order is one less, i.e. 17. For the 16Kb default
>page size this gives you a maximum page size of 2Gb, which isn't
>particularly useful on ia64 since that size is not supported. The only
>reason I can see 18 being useful would be to allow for 1Gb huge pages
>on a system where PAGE_SIZE is set to 8Kb.
>
>Anyway, with the default page size being 16K, it seems that it would
>be more appropriate for CONFIG_FORCE_MAX_ZONEORDER to be either 17 (for
>a 1 Gb max huge page size) or 19 (to support the maximum 4 Gb page size
>available on Itanium 2 systems).
>
>Is there another consideration that I've missed?
>
>John Marvin
>-
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>
>  
>


-- 

	Sincères salutations.


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