Messages in this thread |  | | | Date | Thu, 31 May 2012 15:09:01 -0400 | | From | KOSAKI Motohiro <> | | Subject | Re: [RFC Patch] fs: implement per-file drop caches |
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(5/31/12 8:11 AM), Cong Wang wrote: > On Thu, 2012-05-31 at 02:30 -0400, KOSAKI Motohiro wrote: >> (5/31/12 2:20 AM), Cong Wang wrote: >>> On Wed, 2012-05-30 at 16:14 +0100, Pádraig Brady wrote: >>>> On 05/30/2012 02:38 PM, Cong Wang wrote: >>>>> This is a draft patch of implementing per-file drop caches. >>>>> >>>>> It introduces a new fcntl command F_DROP_CACHES to drop >>>>> file caches of a specific file. The reason is that currently >>>>> we only have a system-wide drop caches interface, it could >>>>> cause system-wide performance down if we drop all page caches >>>>> when we actually want to drop the caches of some huge file. >>>> >>>> This is useful functionality. >>>> Though isn't it already provided with POSIX_FADV_DONTNEED? >>> >>> Thanks for teaching this! >>> >>> However, from the source code of madvise_dontneed() it looks like it is >>> using a totally different way to drop page caches, that is to invalidate >>> the page mapping, and trigger a re-mapping of the file pages after a >>> page fault. So, yeah, this could probably drop the page caches too (I am >>> not so sure, haven't checked the code in details), but with my patch, it >>> flushes the page caches directly, what's more, it can also prune >>> dcache/icache of the file. >> >> madvise should work. I don't think we need duplicate interface. Moreomover >> madvise(2) is cleaner than fcntl(2). >> > > I think madvise(DONTNEED) attacks the problem in a different approach, > it munmaps the file mapping and by the way drops the page caches, my > approach is to drop the page caches directly similar to what sysctl > drop_caches. > > What about private file mapping? Could madvise(DONTNEED) drop the page > caches too even when the other process is doing the same private file > mapping? At least my patch could do this.
Right. But a process can makes another mappings if a process have enough permission. and if it doesn't, a process shouldn't be able to drop a shared cache.
> I am not sure if fcntl() is a good interface either, this is why the > patch is marked as RFC. :-D
But, if you can find certain usecase, I'm not against anymore.
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