Re: PAE and PSE ??

H. Peter Anvin
2006-10-22T23:37:51Z

Rafael J. Wysocki wrote:
> Hi,
>
> On Sunday, 22 October 2006 08:29, Sandeep Kumar wrote:
>> Hi all,
>> I have read in UTLK by bovet that the linux kernel does not uses the
>> PSE bit on an x86
>> machine. Then how come we have the hugetlbfs, which provides support
>> for 4MB pages ?
>
> AFAIK, PSE is only used when PAE is not set and then it enables the 4 MB
> pages. If PAE is set, the 4 MB pages are impossible because there are only
> 512 entries per page table, but 2 MB pages can be used instead (and you don't
> need to set PSE to use them).
>

You're wrong.

PSE refers to 4 MB pages when PAE is not used, and 2 MB pages when PAE
is used.

In theory, you could have PAE without PSE, which would only support 4K
pages.

Linux uses PSE; it may or may not use PAE depending on the configuration.

-hpa