Re: [Ksummit-2010-discuss] checkpoint-restart: naked patch

[Oren Laadan]

On 11/05/2010 01:17 PM, Gene Cooperman wrote:
> On Fri, Nov 05, 2010 at 04:57:33AM -0700, Luck, Tony wrote:
>>> Oren noted that sometimes it's important to stop the process only
>>> for a few milliseconds while one checkpoints. In DMTCP, we do that
>>> by configuring with --enable-forked-checkpointing. This causes us
>>> to fork a child process taking advantage of copy-on-write and then
>>> checkpoint the memory pages of the child while the parent continues
>>> to execute.
>>
>> Interesting ... but while the process is only stopped for the duration
>> of the fork, it may be taking COW faults on almost every page it
>> touches.  I think this will not work well for large HPC applications
>> that allocate most of physical memory as anonymous pages for the
>> application. It may even result in an OOM kill if you don't complete
>> the checkpoint of the child and have it exit in a timely manner.
>>
>> -Tony
>>
> 
> I agree with you that forked checkpointing is probably not what you
> want in the middle of an HPC computation.  But isn't that part of
> the nature of COW?  Whether the COW is invoked within the kernel,
> or from outside the kernel via fork --- in either case, when you have
> mostly dirty pages, you will have to copy most of the pages.
> Do I understand your point correctly?			Thanks,
> 							- Gene

COW is one way of reducing down time (whether through fork or
in-kernel checkpoint). However, it is possible to avoid using
it (and thus avoid extra page faults and memory overload) by
using the page-table "dirty" bit to track dirty pages. This way
one can "pre-copy" the checkpoint image while the application is
running, without additional overhead (the idea is similar to how
live-migration is done).

Oren.
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