* mount: readdir enters a child in the inode table only when it takes a reference
Only readdirplus into the kernel takes a reference on the children it
reports, and only that reference brings a FORGET later to take the entry
back out. Every other listing was inserting all its children anyway.
On WinFsp that meant a listing looked each child up, took a reference,
and immediately gave it back, so a walk of a wide directory paid three
write-lock acquisitions per entry to leave the table exactly as it found
it. On a plain kernel readdir nothing gives the entry back at all, so
listing a directory of 200k files grew both maps by 200k entries that
were never reclaimed.
A dirent's inode number is informational either way: the kernel must
LOOKUP before it can use a nodeid, and the WinFsp adapter re-resolves
every operation by path. So report the number and let the mapping be
built when something actually looks the entry up.
* mount: take the readdirplus reference without a second full lookup
The entry has just been resolved a few lines above, so redoing the whole
lookup only rebuilds the child path and walks both maps again to reach a
counter. Bump it directly, falling back to the full lookup if a Forget
removed the entry in between.
* mount: benchmark a readdir over a 200k directory
Drives doReadDirectory against a meta cache holding 200k entries, one
round of 4096 at a time, for the three front ends that behave
differently: a plain kernel readdir, kernel readdirplus, and a WinFsp
listing that gets attributes but never returns a reference. Reports what
each leaves behind in the inode table alongside the usual metrics.
The sink declares TakesLookupRef as an ordinary method rather than
through the interface, so the same file runs unchanged against an older
tree for comparison.
* mount: stamp an inode on the benchmark's entries
The filer stores one on every entry it writes, so a real listing arrives
with an inode and never derives its own. Leaving it zero made every child
in the benchmark fall through to the MD5 in AsInode, work no filer-backed
mount does, and charged it to both sides of the comparison.
Chris LuandGitHub
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