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

[Paul Turner]

On Wed, Oct 31, 2012 at 5:50 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> Hello,
>
> On Wed, Oct 31, 2012 at 02:59:07PM -0700, Paul Turner wrote:
>> On Wed, Oct 31, 2012 at 2:35 PM, Andrew Morton
>> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>> >
>> > On Tue, 30 Oct 2012 10:29:54 +0900
>> > Minchan Kim <minchan@xxxxxxxxxx> wrote:
>> >
>> > > This patch introudces new madvise behavior MADV_VOLATILE and
>> > > MADV_NOVOLATILE for anonymous pages. It's different with
>> > > John Stultz's version which considers only tmpfs while this patch
>> > > considers only anonymous pages so this cannot cover John's one.
>> > > If below idea is proved as reasonable, I hope we can unify both
>> > > concepts by madvise/fadvise.
>> > >
>> > > Rationale is following as.
>> > > Many allocators call munmap(2) when user call free(3) if ptr is
>> > > in mmaped area. But munmap isn't cheap because it have to clean up
>> > > all pte entries and unlinking a vma so overhead would be increased
>> > > linearly by mmaped area's size.
>> >
>> > Presumably the userspace allocator will internally manage memory in
>> > large chunks, so the munmap() call frequency will be much lower than
>> > the free() call frequency.  So the performance gains from this change
>> > might be very small.
>>
>> I don't think I strictly understand the motivation from a
>> malloc-standpoint here.
>>
>> These days we (tcmalloc) use madvise(..., MADV_DONTNEED) when we want
>> to perform discards on Linux.    For any reasonable allocator (short
>> of binding malloc --> mmap, free --> unmap) this seems a better
>> choice.
>>
>> Note also from a performance stand-point I doubt any allocator (which
>> case about performance) is going to want to pay the cost of even a
>> null syscall about typical malloc/free usage (consider: a tcmalloc
>
> Good point.
>
>> malloc/free pairis currently <20ns).  Given then that this cost is
>> amortized once you start doing discards on larger blocks MADV_DONTNEED
>> seems a preferable interface:
>> - You don't need to reconstruct an arena when you do want to allocate
>> since there's no munmap/mmap for the region to change about
>> - There are no syscalls involved in later reallocating the block.
>
> Above benefits are applied on MADV_VOLATILE, too.
> But as you pointed out, there is a little bit overhead than DONTNEED
> because allocator should call madvise(MADV_NOVOLATILE) before allocation.
> For mavise(NOVOLATILE) does just mark vma flag, it does need mmap_sem
> and could be a problem on parallel malloc/free workload as KOSAKI pointed out.
>
> In such case, we can change semantic so malloc doesn't need to call
> madivse(NOVOLATILE) before allocating. Then, page fault handler have to
> check whether this page fault happen by access of volatile vma. If so,
> it could return zero page instead of SIGBUS and mark the vma isn't volatile
> any more.

I think being able to determine whether the backing was discarded
(about a atomic transition to non-volatile) would be a required
property to make this useful for non-malloc use-cases.

>
>>
>> The only real additional cost is address-space.  Are you strongly
>> concerned about the 32-bit case?
>
> No. I believe allocators have a logic to clean up them once address space is
> almost full.
>
> Thanks, Paul.
>
> --
> Kind regards,
> Minchan Kim
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