tmpfs - Fix periodic syncer cpu-bound stalls w/tmpfs on big-mem boxes
* A bug in tmpfs leaves dirty tmpfs vnodes (which is basically all
of them) on the syncer list, causing the syncer to scan the entire
list every few seconds. When a large number of vnodes are involved,
this can lead to noticable cpu-bound stalls on the cpu the syncer is
running on.
* Generally speaking this should significantly improve applications
which use tmpfs a lot, such as synth runs, but was probably not noticed
on machines with less than 32GB of ram due to the lower kern.maxvnodes
default.
* Solved by unconditionally removing the vnode from the syncer list
in tmpfs_fsync(), but otherwise leaving the vnode marked VISDIRTY and
usually also VOBJDIRTY. Tmpfs's reclaim code properly handles the
disconnect regardless of the syncer state of the vnode.
Removing the vnode ensures that only one syncer pass is performed on
it, instead of repeated passes every few seconds.
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