Messages in this thread | | | Subject | Re: Better pagecache statistics ? | From | Badari Pulavarty <> | Date | Thu, 01 Dec 2005 09:15:15 -0800 |
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On Thu, 2005-12-01 at 15:08 -0200, Marcelo Tosatti wrote: > On Thu, Dec 01, 2005 at 05:10:11PM +0100, Arjan van de Ven wrote: > > > Out of "Cached" value - to get details like > > > > > > <mmap> - xxx KB > > > <shared mem> - xxx KB > > > <text, data, bss, malloc, heap, stacks> - xxx KB > > > <filecache pages total> -- xxx KB > > > (filename1 or <dev>, <ino>) -- #of pages > > > (filename2 or <dev>, <ino>) -- #of pages > > > > > > This would be really powerful on understanding system better. > > > > to some extend it might be useful. > > I have a few concerns though > > 1) If we make these stats into an ABI then it becomes harder to change > > the architecture of the VM radically since such concepts may not even > > exist in the new architecture. As long as this is some sort of advisory, > > humans-only file I think this isn't too much of a big deal though. > > > > 2) not all the concepts you mention really exist as far as the kernel is > > concerned. I mean.. a mmap file is file cache is .. etc. > > malloc/heap/stacks are also not differentiated too much and are mostly > > userspace policy (especially thread stacks). > > > > A split in > > * non-file backed > > - mapped once > > - mapped more than once > > * file backed > > - mapped at least once > > - not mapped > > I can see as being meaningful. Assigning meaning to it beyond this is > > dangerous; that is more an interpretation of the policy userspace > > happens to use for things and I think coding that into the kernel is a > > mistake. > > > > Knowing which files are in memory how much is, as debug feature, > > potentially quite useful for VM hackers to see how well the various VM > > algorithms work. I'm concerned about the performance impact (eg you can > > do it only once a day or so, not every 10 seconds) and about how to get > > this data out in a consistent way (after all, spewing this amount of > > debug info will in itself impact the vm balances) > > Most of the issues you mention are null if you move the stats > maintenance burden to userspace. > > The performance impact is also minimized since the hooks > (read: overhead) can be loaded on-demand as needed. >
The overhead is - going through each mapping/inode in the system and dumping out "nrpages" - to get per-file statistics. This is going to be expensive, need locking and there is no single list we can traverse to get it. I am not sure how to do this.
Thanks, Badari
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