Re: wrong madvise(MADV_DONTNEED) semantic

[Darren Hart <darren@dvhart.com>]

Jörn Engel wrote:
> On Tue, 28 June 2005 16:05:11 -0400, Robert Love wrote:
> 
>>I like the idea (I think someone suggested this early on) of renaming
>>the current MADV_DONTNEED to MADV_FREE and then adding a correct
>>MADV_DONTNEED.
> 
> 
> Imo, that's still a crime against common sense.  Madvice should give
> the kernel some advice about which data to keep or not to keep in
> memory, hence the name.  It should *not* tell the kernel to corrupt
> data, which currently appears to be the case.
> 
> If the application knows 100% that it is the _only_ possible user of
> this data and will never again use it, dropping dirty pages might be a
> sane option.  Effectively that translates to anonymous memory only.
> In all other cases, dirty pages should be written back.

There is also the case of shmget/shmat memory segments.  Some 
applications will use these in order to map a very large amount of 
memory and then madvise(MADV_DONTNEED) in order to play nice with the 
rest of the system should memory pressure / system load / etc require 
it.  Obviously if other tasks have these segments mapped, the pages 
cannot be discarded.  If a task is the sole "mapper" of the region and 
doesn't need that memory (ever again) it would be good to avoid the i/o 
overhead of swapping it out and just discarding it.  Perhaps 
MADV_DONTNEED isn't the right place for this, but there is demand for 
this behavior.

--Darren Hart

> 
> 
>>And, as I said, the man page needs clarification.
> 
> 
> Definitely.
> 
> Jörn
>