Re: [PATCH] make MADV_FREE lazily free memory

Eric Dumazet
2007-04-12T05:44:59Z

Rik van Riel a A(C)crit :
> Eric Dumazet wrote:
>> Rik van Riel a A(C)crit :
>>> Make it possible for applications to have the kernel free memory
>>> lazily. This reduces a repeated free/malloc cycle from freeing
>>> pages and allocating them, to just marking them freeable. If the
>>> application wants to reuse them before the kernel needs the memory,
>>> not even a page fault will happen.
>
>> I dont understand this last sentence. If not even a page fault
>> happens, how the kernel knows that the page was eventually reused by
>> the application, and should not be freed in case of memory pressure ?
>
> Before maybe freeing the page, the kernel checks the referenced
> and dirty bits of the page table entries mapping that page.
>
>> ptr = mmap(some space);
>> madvise(ptr, length, MADV_FREE);
>> /* kernel may free the pages */
>
> All this call does is:
> - clear the accessed and dirty bits
> - move the page to the far end of the inactive list,
> where it will be the first to be reclaimed
>
>> sleep(10);
>>
>> /* what the application must do know before reusing space ? */
>> memset(ptr, data, 10000);
>> /* kernel should not free ptr[0..10000] now */
>
> Two things can happen here.
>
> If this program used the pages before the kernel needed
> them, the program will be reusing its old pages.

ah ok, this is because accessed/dirty bits are set by hardware and not a page
fault. Is it true for all architectures ?

>
> If the kernel got there first, you will get page faults
> and the kernel will fill in the memory with new pages.

perfect

>
> Both of these alternatives are transparent to userspace.
>

Thanks a lot for these clarifications. This will fly :)