Re: [PATCH 0/5] Volatile Ranges (v12) & LSF-MM discussion fodder

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Apr 01, 2014 at 09:12:44PM -0700, John Stultz wrote:
> On 04/01/2014 04:01 PM, Dave Hansen wrote:
> > On 04/01/2014 02:35 PM, H. Peter Anvin wrote:
> >> On 04/01/2014 02:21 PM, Johannes Weiner wrote:
> >>> Either way, optimistic volatile pointers are nowhere near as
> >>> transparent to the application as the above description suggests,
> >>> which makes this usecase not very interesting, IMO.
> >> ... however, I think you're still derating the value way too much.  The
> >> case of user space doing elastic memory management is more and more
> >> common, and for a lot of those applications it is perfectly reasonable
> >> to either not do system calls or to have to devolatilize first.
> > The SIGBUS is only in cases where the memory is set as volatile and
> > _then_ accessed, right?
> Not just set volatile and then accessed, but when a volatile page has
> been purged and then accessed without being made non-volatile.
> 
> 
> > John, this was something that the Mozilla guys asked for, right?  Any
> > idea why this isn't ever a problem for them?
> So one of their use cases for it is for library text. Basically they
> want to decompress a compressed library file into memory. Then they plan
> to mark the uncompressed pages volatile, and then be able to call into
> it. Ideally for them, the kernel would only purge cold pages, leaving
> the hot pages in memory. When they traverse a purged page, they handle
> the SIGBUS and patch the page up.

How big are these libraries compared to overall system size?

> Now.. this is not what I'd consider a normal use case, but was hoping to
> illustrate some of the more interesting uses and demonstrate the
> interfaces flexibility.

I'm just dying to hear a "normal" use case then. :)

> Also it provided a clear example of benefits to doing LRU based
> cold-page purging rather then full object purging. Though I think the
> same could be demonstrated in a simpler case of a large cache of objects
> that the applications wants to mark volatile in one pass, unmarking
> sub-objects as it needs.

Agreed.

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