Re: [PATCH] make MADV_FREE lazily free memory

[Nick Piggin <nickpiggin@yahoo.com.au>]

Eric Dumazet wrote:
> Rik van Riel a ecrit :
> 
>> Eric Dumazet wrote:
>>
>>> Rik van Riel a ecrit :
>>>
>>>> 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.

No it isn't.

> Is it true for all architectures ?

No.

-- 
SUSE Labs, Novell Inc.

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