We just turned delegations on for two big NFS servers. One characteristic of our site is that we have lots of small files and lots of files open. On one server, CPU in system state went to 30%, and NFS performance ground to a halt. When I disabled delegations it came back. The other server was showing high CPU on nfsd, but not enough to disable the server, so I looked around. The server where delegations are still on is spending most of its time in nfsd_file_lru_cb. That's not the case with the server where we've disabled delegations. Here's a typical perf top Overhead Shared Object Symbol 44.87% [kernel] [k] __list_lru_walk_one 13.18% [kernel] [k] native_queued_spin_lock_slowpath.part.0 7.24% [kernel] [k] nfsd_file_lru_cb 2.61% [kernel] [k] sha1_transform 0.99% [kernel] [k] __crypto_alg_lookup 0.95% [kernel] [k] _raw_spin_lock 0.89% [kernel] [k] memcpy_erms 0.77% [kernel] [k] mutex_lock 0.65% [kernel] [k] svc_tcp_recvfrom I looked at the code. I'm not clear whether there's a problem with GC'ing the entries, or it's just being called too often (maybe a table is too small?) When I disabled delegations, it immediately stopped spending all that time in nfsd_file_lru_cb. The number of delegations starting going down slowly. I suspect our system needs a lot more delegations than the maximum table size, and it's thrashing. The sizes were about 40,000 and 60,000 on the two machines. Systems are 384 G and 768 G, respectively. The maximum number of delegations is smaller than I would have expected based on comments in the code.
