On Tue, Jan 14, 2025 at 6:55 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > On Tue, Jan 14, 2025 at 4:38 AM Shyam Prasad N <nspmangalore@xxxxxxxxx> wrote: > > > > The Linux kernel does buffered reads and writes using the page cache > > layer, where the filesystem reads and writes are offloaded to the > > VM/MM layer. The VM layer does a predictive readahead of data by > > optionally asking the filesystem to read more data asynchronously than > > what was requested. > > > > The VFS layer maintains a dentry cache which gets populated during > > access of dentries (either during readdir/getdents or during lookup). > > This dentries within a directory actually forms the address space for > > the directory, which is read sequentially during getdents. For network > > filesystems, the dentries are also looked up during revalidate. > > > > During sequential getdents, it makes sense to perform a readahead > > similar to file reads. Even for revalidations and dentry lookups, > > there can be some heuristics that can be maintained to know if the > > lookups within the directory are sequential in nature. With this, the > > dentry cache can be pre-populated for a directory, even before the > > dentries are accessed, thereby boosting the performance. This could > > give even more benefits for network filesystems by avoiding costly > > round trips to the server. > > > > I believe you are referring to READDIRPLUS, which is quite common > for network protocols and also supported by FUSE. This discussion is not completely about readdirplus, but definitely is a part of it. I'm suggesting doing the next set of readdir() calls in advance, so that the data needed to serve those are already in the cache. I'm also suggesting artificially doing a readdir to avoid sequential revalidation of each dentry; or a readdirplus to avoid stat of each inode corresponding to these dentries > > Unlike network protocols, FUSE decides by server configuration and > heuristics whether to "fuse_use_readdirplus" - specifically in readdirplus_auto > mode, FUSE starts with readdirplus, but if nothing calls lookup on the > directory inode by the time the next getdents call, it stops with readdirplus. > > I personally ran into the problem that I would like to control from the > application, which knows if it is doing "ls" or "ls -l" whether a specific > getdents() will use FUSE readdirplus or not, because in some situations > where "ls -l" is not needed that can avoid a lot of unneeded IO. > > I do not know if implementing readdirplus (i.e. populate inode and dentry) > makes sense for disk filesystems, but if we do it in VFS level, there has to > be at an API to control or at least opt-out of readdirplus, like with readahead. That would be a great knob to have for network filesystems. We have to rely on heuristics today to predict which of these patterns the workload is using. > > Thanks, > Amir. -- Regards, Shyam
