Specifically regarding the behavior in different kernels, from `man posix_fadvise`: "In kernels before 2.6.6, if len was specified as 0, then this was interpreted literally as "zero bytes", rather than as meaning "all bytes through to the end of the file"."
On Oct 18, 2016, at 8:57 AM, Michael Kjellman <mkjell...@internalcircle.com<mailto:mkjell...@internalcircle.com>> wrote: Right, so in SSTableReader#GlobalTidy$tidy it does: // don't ideally want to dropPageCache for the file until all instances have been released CLibrary.trySkipCache(desc.filenameFor(Component.DATA), 0, 0); CLibrary.trySkipCache(desc.filenameFor(Component.PRIMARY_INDEX), 0, 0); It seems to me every time the reference is released on a new sstable we would immediately tidy() it and then call posix_fadvise with POSIX_FADV_DONTNEED with an offset of 0 and a length of 0 (which I'm thinking is doing so in respect to the API behavior in modern Linux kernel builds?). Am I reading things correctly here? Sorta hard as there are many different code paths the reference could have tidy() called. Why would we want to drop the segment we just write from the page cache -- wouldn't that most likely be the most hot data, and even if it turned out not to be wouldn't it be better in this case to have kernel be smart at what it's best at? best, kjellman On Oct 18, 2016, at 8:50 AM, Jake Luciani <jak...@gmail.com<mailto:jak...@gmail.com>> wrote: The main point is to avoid keeping things in the page cache that are no longer needed like compacted data that has been early opened elsewhere. On Oct 18, 2016 11:29 AM, "Michael Kjellman" <mkjell...@internalcircle.com<mailto:mkjell...@internalcircle.com>> wrote: We use posix_fadvise in a bunch of places, and in stereotypical Cassandra fashion no comments were provided. There is a check the OS is Linux (okay, a start) but it turns out the behavior of providing a length of 0 to posix_fadvise changed in some 2.6 kernels. We don't check the kernel version -- or even note it. What is the *expected* outcome of our use of posix_fadvise -- not what does it do or not do today -- but what problem was it added to solve and what's the expected behavior regardless of kernel versions. best, kjellman Sent from my iPhone
