Messages in this thread | | | Date | Fri, 18 May 2012 11:44:39 +0800 | From | Nai Xia <> | Subject | Re: [patch 0/5] refault distance-based file cache sizing |
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On 2012/05/18 05:08, Johannes Weiner wrote: > On Wed, May 16, 2012 at 08:56:54PM +0800, nai.xia wrote: >> On 2012/05/16 14:51, Johannes Weiner wrote: >>> There may have been improvements from clock-pro, but it's hard to get >>> code merged that does not behave as expected in theory with nobody >>> understanding what's going on. > > Damn, that sounded way harsher and arrogant than I wanted it to sound. > And it's only based on what I gathered from the discussions on the > list archives. Sorry :(
No harm done, man. I just understood your words in this way. :)
But I do think that Clock-pro deserves its credit, since after all it's that research work firstly brought the idea of "refault/reuse distance" to the kernel community. Further more, it's also good to let the researchers and the community to together have some brain-storm of this problem if it's really hard to deal with in reality.
> >> OK, I assume that you do aware that the system you constructed with >> this simple and understandable idea looks like a so called "feedback >> system"? Or in other words, I think theoretically the refault-distance >> of a page before and after your algorithm is applied is not the same. >> And this changed refault-distance pattern is then feed as input into >> your algorithm. A feedback system may be hard(and may be simple) to >> analyze but may also work well magically. > > I'm with you on that, but I can't see an alternative in this case. We
I trend to agree, I once tried to deal with an anti-LRU pattern(e.g. the big loop like you said) of a app from kernel space and failed. Seems it's hard to gather a very accurate information of a program's real memory footprint in mixed workloads with only the help of pte bits...(but also may due to my lack of skills in tweaking the reclaiming code...)
> can't predict future page accesses very well, so we have to take > speculative shots and be considerate about the consequences. > > And BECAUSE we may get it wrong, the algorithm does not rely on the > decisions it makes to be correct. For example, it does not activate > pages based on refault distance, but requires the refaulted page to > win the race against an actual active page. Likewise, pages are not > evicted from the active list directly, instead they get a chance at > re-activation when challenged.
Yes. That sounds a smart handling.
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