Re: [RFC v5 0/8] Support volatile for anonymous range

[Sanjay Ghemawat]

On Wed, Jan 2, 2013 at 8:27 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> This is still RFC because we need more input from user-space
> people, more stress test, design discussion about interface/reclaim

Speaking as one of the authors of tcmalloc, I don't see any particular
need for this new system call for tcmalloc.  We are fine using
madvise(MADV_DONTNEED) and don't notice any significant
performance issues caused by it.  Background: we throttle how
quickly we release memory back to the system (1-10MB/s), so
we do not call madvise() very much, and we don't end up reusing
madvise-ed away pages at a fast rate. My guess is that we won't
see large enough application-level performance improvements to
cause us to change tcmalloc to use this system call.

> - What's different with madvise(DONTNEED)?
>
>   System call semantic
>
>   DONTNEED makes sure user always can see zero-fill pages after
>   he calls madvise while mvolatile can see old data or encounter
>   SIGBUS.

Do you need a new system call for this?  Why not just a new flag to madvise
with weaker guarantees than zero-filling?  All of the implementation changes
you point out below could be triggered from that flag.
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