Why kernel.org is slow

Why kernel.org is slow

Linux appears to do a much better job than the NT kernel for
the in memory data structures. A cheap way to see this is to
run git on a windows system. There is an order of magnitude
performance hit for directory sensitive things. I don't believe
that is just cygwin.

ext3 for large directories hashes the filename and looks it up in a
btree. Using a hash of the filename results in a better branching
factor in your btree. So the on-disk data structures are not at
a disadvantage.

I haven't looked but ext2+ directories should all be kept in the same
block group which is roughly a single disk track. So even with
fragmentation the disk track cache should work well. I don't remember
if block groups are small enough so that they always map to the
same disk track though.

So I'm pretty certain the issue is the large directories the inode
semaphore.

Read-ahead should help a lot if the pages don't get thrown out before we
use them.

Changing the locking to allow more concurrency is a trickier problem.
If done right my gut feel is that you should be able to operate
essentially lock free, with multiple concurrent writes and reads going on
simultaneously. The readdir semantics allow for it. But anything with a
high degree of concurrency comes with tricky corner cases.

Eric