(from Google Summer of Code 2008 project listing)

Prior to UFS2, the performance of name lookups in directories with many entries on FreeBSD could be quite slow. FFS does not maintain an on-disk index of directory entries, so each name lookup required a linear search of the directory entries. With UFS2 a new technique called dirhash was developed to generate a hash map in memory of directory entries the first time a name lookup is run on a large directory, so that subsequent name lookups will be much faster. Dirhash has had a substantial positive impact on filesystem performance.

However, the amount of memory which can be used by dirhashes is fixed by the kernel (although it is tunable at runtime via a sysctl). I propose to implement dynamic memory allocation for dirhashes, allowing more memory to be allocated for dirhash when needed if it is available, and reclaimed if memory becomes scarce. The goal of this is to improve performance for filesystems with very large directories on systems with memory to spare, without the user having to manually tune the amount of memory available for dirhash.

Furthermore, if time permits I would like to investigate implementing on-disk indexing for UFS2. If directory indices are generated and stored on disk, there would no longer be a need to generate a completely new dirhash each time a directory is first accessed after system startup, or after a dirhash for a directory not recently accessed has been disposed of to conserve memory.

• Either change dirhash memory allocation to use buffer cache, or keep existing memory allocation model but register new callback function to free older dirhashes when vm_lowmem events occur.
• Once code works reasonably well in my own tests, call for testing from the community to shake out more bugs if possible.
• Test for performance differences with dynamic dirhash vs. static under different work loads (e.g. huge maildirs, subversion fsfs repositories with a great many revisions, different amounts of memory in machine, etc.)
• Get code in a state suitable for being committed to -CURRENT
• (Time permitting) Begin work on on-disk directory indexing in UFS2.

I blog about this project here. The code I am working on can be found in the FreeBSD Perforce depot here.

(2008-6-19) I am working on implementing a function to be registered with EVENTHANDLER_REGISTER() for vm_lowmem events that will free dirhashes. I am trying to determine how to decide which hashes to free, and will need to find how many should be freed each time this event occurs. Determining the latter might be easiest through trial and error, watching how often vm_lowmem events pop up when the system is low on memory.

(2008-6-27) Here is a patch with a first attempt at freeing dirhashes when a low memory signal is sent: dirhash_lowmem_event_2008-6-28.patch. In this patch just the one oldest dirhash that can be freed will be.

(2008-7-12) I now have an updated patch: attachment:dirhash_lowmem_event_2008-7-12.patch. Now the dirhash data structure has a new field for the last used time stamp. When a vm_lowmem event occurs, every dirhash that has not been used for five seconds (this will probably need to be tuned) is discarded. If all dirhashes have been used more recently than this, then the first one on the dirhash list TAILQ will be deleted.