Sounds like a refinement that would be useful to quite a few people. If
someone cares to implement it, I'd be glad to consider a patch. Main issue is
going to be making sure it compiles on all platforms and does something
reasonable on as many as possible.
- Benjamin
On Apr 26, 2010, at 5:45 AM, Sylvain Le Gall wrote:
> Hello,
>
> On Mon, Apr 26, 2010 at 10:17:49AM +0100, Enrico Zini wrote:
>> My laptop has 2GB of RAM, but sometimes I want to sync directory trees
>> with a similar order of magnitude of data in them. When unison scans the
>> local data, linux has a tendency to swap out running application to free
>> RAM for caching the data that unison reads. The result is that while
>> unison is running, my laptop is thrashing heavily as the kernel loads
>> back from swap those apps that it just swapped.
>>
>> I'm running with default kernel VM settings. Running unison under
>> nocache (http://www.enricozini.org/2010/tips/nocache/) does not cause
>> the thrashing and keeps the system perfectly usable during syncs, if a
>> bit slower I/O wise, which is perfectly reasonable.
>>
>> I am of course the first to agree that the problem in this is not in
>> unison but in Linux itself. The operating system should not swap out
>> running apps: this behaviour is preposterous, and everytime I experience
>> it a part of my soul dies drowned by its own tears.
>>
>> It might still make sense, however, to have an option in unison
>> (disabled by default) that turns on fadvise cache hinting. One would
>> turn it on when syncing big, seldom accessed archives like photo
>> galleries or music collections, and keep it off when syncing the home
>> dir with files that are accessed often.
>>
>> Even more ideal would be to be able to turn it on/off on a per-directory
>> basis, so that I can sync my home dir and have the system cache
>> everything except pictures, music and videos.
>>
>
> I played a bit with fadvise and I even have an OCaml binding for it,
> somewhere on my hard-drive. I think it can be quite efficient but you
> should talk about this directly with upstream.
>
> I forward this mail to upstream, so that you can discuss it.
>
> The original bug is here:
> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=579213
>
> Regards
> Sylvain Le Gall
>
>
>
>
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