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

[John Stultz <john.stultz@linaro.org>]

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.

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.

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.

thanks
-john