On 07/16/2018 05:09 AM, Michal Hocko wrote: > On Fri 13-07-18 10:36:14, Dave Chinner wrote: > [...] >> By limiting the number of negative dentries in this case, internal >> slab fragmentation is reduced such that reclaim cost never gets out >> of control. While it appears to "fix" the symptoms, it doesn't >> address the underlying problem. It is a partial solution at best but >> at worst it's another opaque knob that nobody knows how or when to >> tune. > Would it help to put all the negative dentries into its own slab cache? > >> Very few microbenchmarks expose this internal slab fragmentation >> problem because they either don't run long enough, don't create >> memory pressure, or don't have access patterns that mix long and >> short term slab objects together in a way that causes slab >> fragmentation. Run some cold cache directory traversals (git >> status?) at the same time you are creating negative dentries so you >> create pinned partial pages in the slab cache and see how the >> behaviour changes.... > Agreed! Slab fragmentation is a real problem we are seeing for quite > some time. We should try to address it rather than paper over it with > weird knobs. I am aware that you don't like the limit knob that control how many negative dentries are allowed as a percentage of total system memory. I got comments in the past about doing some kind of auto-tuning. How about consolidating the 2 knobs that I currently have in the patchset into a single one with 3 possible values, like: 0 - no limiting 1 - set soft limit to "a constant + 4 x max # of positive dentries" and warn if exceeded 2 - same limit but kill excess negative dentries after use. Does that kind of knob make more sense to you? Cheers, Longman
