Re: [RFC] madvise(MADV_TRUNCATE)

[Badari Pulavarty <pbadari@us.ibm.com>]

Theodore Ts'o wrote:

> This is somewhat related to something which the JVM folks have been
> pestering us (i.e., anyone within the LTC who will listen :-) for a
> while now, which is a way to very _quickly_ (i.e., faster than munmap)
> tell the kernel that a certain range of pages are not used any more by
> the JVM, because the garbage collector has finished, and the indicated
> region of memory is unused "oldspace".  
> 
> If those pages are needed the kernel is free to grab them for an other
> purpose without writing them back to swap, and any attempt to read
> from said memory afterwards should result in undefined behaviour.  In
> practice, the JVM should never (absent bugs) try to read or write from
> such pages before it tells the kernel that it cares about a region of
> memory again (i.e., when the garbage collector runs again and needs to
> use that section of memory for memory allocations, at which point it
> won't care what the old memory values).
> 
> The JVM folks have tried using munmap, but it's too slow and if the
> system isn't under memory pressure (as would be the case when an
> application is correctly tuned for the machine and in benchmark
> situations :-), completely unnecessary, since the pages will have to
> mmaped back in after the next GC anyway.  So currently today, the JVM
> folks simply do not release oldspace memory back to the system at all
> after a GC.
> 
> What would be nice would be there is some way that an VMA could be
> marked, "contents are unimportant", so that if there is a need for any
> pages, the pages can be assumed to be clean and can simply be reused
> for another purpose once they are deactivated without needing to waste
> any swap bandwidth writing out pages whose contents are unimportant
> and not in use by the JVM.  Then when the region is marked as being in
> use again, and when it is touched, we simply map in the zero page COW.
> 
> That way, if the system is operating with plenty of memory, the
> performance is minimal (simply setting and clearing a bit in the VMA).
> But if the system is under memory pressure, the JVM is being a good
> citizen and allowing its memory pages to be used for other purposes.
> 
> Does this sound like an idea that would be workable?  I'm not a VM
> expert, but it doesn't sound like it's that hard, and I don't see any
> obvious flaws with this plan.

Ted,

Like Andrea mentioned MADV_DONTNEED should be able to do what JVM
folks want. If they want more than that, get in touch with me.
While doing MADV_REMOVE, I will see if I can satsify their needs also.

Thanks,
Badari



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