Re: [RFC v2] Support volatile range for anon vma

[Paul Turner <pjt@google.com>]

On Wed, Oct 31, 2012 at 3:56 PM, KOSAKI Motohiro
<kosaki.motohiro@gmail.com> wrote:
>>> > Allocator should call madvise(MADV_NOVOLATILE) before reusing for
>>> > allocating that area to user. Otherwise, accessing of volatile range
>>> > will meet SIGBUS error.
>>>
>>> Well, why?  It would be easy enough for the fault handler to give
>>> userspace a new, zeroed page at that address.
>>
>> Note: MADV_DONTNEED already has this (nice) property.
>
> I don't think I strictly understand this patch. but maybe I can answer why
> userland and malloc folks don't like MADV_DONTNEED.
>
> glibc malloc discard freed memory by using MADV_DONTNEED
> as tcmalloc. and it is often a source of large performance decrease.
> because of MADV_DONTNEED discard memory immediately and
> right after malloc() call fall into page fault and pagesize memset() path.
> then, using DONTNEED increased zero fill and cache miss rate.
>
> At called free() time, malloc don't have a knowledge when next big malloc()
> is called. then, immediate discarding may or may not get good performance
> gain. (Ah, ok, the rate is not 5:5. then usually it is worth. but not everytime)
>

Ah; In tcmalloc allocations (and their associated free-lists) are
binned into separate lists as a function of object-size which helps to
mitigate this.

I'd make a separate more general argument here:
If I'm allocating a large (multi-kilobyte object) the cost of what I'm
about to do with that object is likely fairly large -- The fault/zero
cost a probably fairly small proportional cost, which limits the
optimization value.

>
> In past, several developers tryied to avoid such situation, likes
>
> - making zero page daemon and avoid pagesize zero fill at page fault
> - making new vma or page flags and mark as discardable w/o swap and
>   vmscan treat it. (like this and/or MADV_FREE)
> - making new process option and avoid page zero fill from page fault path.
>   (yes, it is big incompatibility and insecure. but some embedded folks thought
>    they are acceptable downside)
> - etc
>
>
> btw, I'm not sure this patch is better for malloc because current MADV_DONTNEED
> don't need mmap_sem and works very effectively when a lot of threads case.
> taking mmap_sem might bring worse performance than DONTNEED. dunno.

MADV_VOLATILE also seems to end up looking quite similar to a
user-visible (range-based) cleancache.

A second popular use-case for such semantics is the case of
discardable cache elements (e.g. web browser).  I suspect we'd want to
at least mention these in the changelog.  (Alternatively, what does a
cleancache-backed-fs exposing these semantics look like?)

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