Oct 16, 2009

pefs dircache benchmark

I've recently added directory caching into pefs.

Despite of being directory listing cache (like dirhash for ufs) it also acts as encrypted file name cache. So that there is no need to decrypt names for the same entries all the time. That was really big issue because directory listing has to be reread on almost every vnode lookup operation. It made operations on directories with 1000 and more files too time consuming.

The cache is getting updated at two points: during vnode lookup operation and during readdir call. Vnode generation attribute is used to monitor directory changes (the same way NFS works) and expire the cache if it changes. There is no per-operation monitoring because that would violate stacked filesystem nature (and also complicate the code). There are some issues regarding large directories handling within dircache. First of all results of consequent readdir calls considered inconsistent, i.e cache expires if user provided buffer is too small to fit entire directory listing. And while doing a vnode lookup search doesn't terminate if matching directory entry found, it further traverses directory to update the cache.

There is vfs.pefs.dircache_enable sysctl to control cache validity. Setting it to zero would force always treating cache as invalid, and thus dircache would function only as a file name encryption cache.

At the moment caching is only enabled for name decryption, but there are operations like rm or rmdir which perform name encryption on every call to pass data to underlying filesystem. Enabling caching for such operations is not going to be hard, but I just want code to stabilize a bit before moving further.

I've performed two types of tests: dbench and handling directories with large number of files. I've used pefs mounted on top of tmpfs to measure pefs overhead but not disk io performance. Salsa20 algorithms with 256 bit key was chosen because of being the fastest available. Before each run underlying tmpfs filesystem was remounted. Each test was run for 3 times, and average of results is shown in charts (distribution was less then 2%). Also note that I've used kernel with some extra debugging compiled in (invariants, lock debugging).

dbench doesn't show much difference with dircache enable comparing to plain pefs and old pefs without dircache: 143,635 Mb/s against 116,746 Mb/s; although, it's 18% improvement witch is very good imho. Also interesting is that result gets just a bit lower after setting vfs.pefs.dircache_enable=0: 141,289 Mb/s with dircache_enable=0 against 143,635 Mb/s.

Dbench uses directories with small number of entries (usually ~20). That perfectly explains the results achieved. Handling large directories is where dircache shines. I've used the following trivial script for testing, it creates 1000 or 2000 files, does 'ls -l' and removes these files:
for i in `jot 1000`; do
touch test-$i
ls -Al >/dev/null
find . -name test-\* -exec rm '{}' +

The chart speaks for itself. And per file overhead looks much closer to expected linear growth after running the same test for 3000 files:

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