On Thu, Feb 27, 2020 at 07:34:12PM -0800, Matthew Wilcox wrote: > On Thu, Feb 27, 2020 at 05:55:43PM +0800, Ian Kent wrote: > > Not all file systems even produce negative hashed dentries. > > > > The most beneficial use of them is to improve performance of rapid > > fire lookups for non-existent names. Longer lived negative hashed > > dentries don't give much benefit at all unless they suddenly have > > lots of hits and that would cost a single allocation on the first > > lookup if the dentry ttl expired and the dentry discarded. > > > > A ttl (say jiffies) set at appropriate times could be a better > > choice all round, no sysctl values at all. > > The canonical argument in favour of negative dentries is to improve > application startup time as every application searches the library path > for the same libraries. Only they don't do that any more: Tell that to scripts that keep looking through $PATH for binaries each time they are run. Tell that to cc(1) looking through include path, etc. Ian, autofs is deeply pathological in that respect; that's OK, since it has very unusual needs, but please don't use it as a model for anything else - its needs *are* unusual.
