> On Dec 3, 2015, at 2:53 PM, Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx> wrote:
>
> Hi Chuck,
>
> On Thu, Dec 3, 2015 at 10:31 AM, Chuck Lever <chuck.lever@xxxxxxxxxx> wrote:
>> Hi-
>>
>> I’m looking into NFS WRITE latency on the Linux NFS server.
>> With a high speed network fabric and ultra-fast storage (think
>> persistent memory) we can get the per-WRITE latency under 50us
>> (as observed on the wire at the server).
>>
>> One source of the remaining latency appears to be nfsd_open.
>> Here is an example of a call graph captured during a simple
>> iozone 4KB direct write workload (v4.4-rc3):
>>
>> 2) | nfsd_open [nfsd]() {
>> 2) | fh_verify [nfsd]() {
>> 2) | rqst_exp_find [nfsd]() {
>> 2) | exp_find [nfsd]() {
>> 2) | exp_find_key [nfsd]() {
>> 2) 0.043 us | svc_expkey_hash [nfsd]();
>> 2) | sunrpc_cache_lookup [sunrpc]() {
>> 2) 0.027 us | _raw_read_lock();
>> 2) | getboottime64() {
>> 2) 0.022 us | ns_to_timespec();
>> 2) 0.210 us | }
>> 2) 0.021 us | get_seconds();
>> 2) 0.854 us | }
>> 2) | cache_check [sunrpc]() {
>> 2) | getboottime64() {
>> 2) 0.021 us | ns_to_timespec();
>> 2) 0.206 us | }
>> 2) 0.022 us | get_seconds();
>> 2) 0.596 us | }
>> 2) 2.050 us | }
>> 2) | exp_get_by_name [nfsd]() {
>> 2) | svc_export_lookup [nfsd]() {
>> 2) | sunrpc_cache_lookup [sunrpc]() {
>> 2) 0.022 us | _raw_read_lock();
>> 2) 0.024 us | svc_export_match [nfsd]();
>> 2) | getboottime64() {
>> 2) 0.023 us | ns_to_timespec();
>> 2) 0.210 us | }
>> 2) 0.022 us | get_seconds();
>> 2) 0.997 us | }
>> 2) 1.181 us | }
>> 2) | cache_check [sunrpc]() {
>> 2) | getboottime64() {
>> 2) 0.022 us | ns_to_timespec();
>> 2) 0.210 us | }
>> 2) 0.022 us | get_seconds();
>> 2) 0.592 us | }
>> 2) 2.147 us | }
>> 2) 4.560 us | }
>> 2) 4.768 us | }
>> 2) | prepare_creds() {
>> 2) | kmem_cache_alloc() {
>> 2) 0.021 us | _cond_resched();
>> 2) 0.233 us | }
>> 2) | security_prepare_creds() {
>> 2) | selinux_cred_prepare() {
>> 2) | kmemdup() {
>> 2) | __kmalloc_track_caller() {
>> 2) 0.022 us | kmalloc_slab();
>> 2) 0.023 us | _cond_resched();
>> 2) 0.448 us | }
>> 2) 0.640 us | }
>> 2) 0.838 us | }
>> 2) 1.129 us | }
>> 2) 1.808 us | }
>> 2) 0.022 us | override_creds();
>> 2) | exportfs_decode_fh() {
>> 2) | shmem_fh_to_dentry() {
>> 2) | ilookup5() {
>> 2) | ilookup5_nowait() {
>> 2) 0.026 us | _raw_spin_lock();
>> 2) | find_inode() {
>> 2) 0.031 us | shmem_match();
>> 2) 0.029 us | _raw_spin_lock();
>> 2) 0.429 us | }
>> 2) 0.826 us | }
>> 2) 0.022 us | _cond_resched();
>> 2) 0.021 us | _cond_resched();
>> 2) 1.417 us | }
>> 2) | d_find_alias() {
>> 2) 0.022 us | _raw_spin_lock();
>> 2) 0.027 us | _raw_spin_lock();
>> 2) 0.423 us | }
>> 2) 0.035 us | iput();
>> 2) 2.410 us | }
>> 2) | find_acceptable_alias() {
>> 2) 0.026 us | nfsd_acceptable [nfsd]();
>> 2) 0.220 us | }
>> 2) 3.032 us | }
>> 2) | nfsd_setuser_and_check_port [nfsd]() {
>> 2) 0.023 us | nfsexp_flags [nfsd]();
>> 2) | nfsd_setuser [nfsd]() {
>> 2) | revert_creds() {
>> 2) | __put_cred() {
>> 2) | call_rcu_sched() {
>> 2) | __call_rcu() {
>> 2) 0.049 us | __call_rcu_nocb_enqueue();
>> 2) 0.265 us | }
>> 2) 0.446 us | }
>> 2) 0.661 us | }
>> 2) 0.854 us | }
>> 2) | prepare_creds() {
>> 2) | kmem_cache_alloc() {
>> 2) 0.022 us | _cond_resched();
>> 2) 0.219 us | }
>> 2) | security_prepare_creds() {
>> 2) | selinux_cred_prepare() {
>> 2) | kmemdup() {
>> 2) | __kmalloc_track_caller() {
>> 2) 0.024 us | kmalloc_slab();
>> 2) 0.022 us | _cond_resched();
>> 2) 0.412 us | }
>> 2) 0.615 us | }
>> 2) 0.801 us | }
>> 2) 0.993 us | }
>> 2) 1.651 us | }
>> 2) 0.033 us | set_groups();
>> 2) 0.023 us | override_creds();
>> 2) 3.323 us | }
>> 2) 0.029 us | nfserrno [nfsd]();
>> 2) 3.919 us | }
>> 2) 0.025 us | check_nfsd_access [nfsd]();
>> 2) 0.022 us | nfsd_permission [nfsd]();
>> 2) + 14.904 us | }
>>
>> So, 15us for fh_verify by itself.
>>
>> 2) | dentry_open() {
>> 2) | get_empty_filp() {
>> 2) | kmem_cache_alloc() {
>> 2) 0.021 us | _cond_resched();
>> 2) 0.325 us | }
>> 2) | security_file_alloc() {
>> 2) | selinux_file_alloc_security() {
>> 2) | kmem_cache_alloc() {
>> 2) 0.022 us | _cond_resched();
>> 2) 0.251 us | }
>> 2) 0.441 us | }
>> 2) 0.633 us | }
>> 2) 0.022 us | __mutex_init();
>> 2) 1.572 us | }
>> 2) | vfs_open() {
>> 2) | do_dentry_open() {
>> 2) | path_get() {
>> 2) 0.025 us | mntget();
>> 2) 0.313 us | }
>> 2) 0.029 us | try_module_get();
>> 2) | security_file_open() {
>> 2) | selinux_file_open() {
>> 2) 0.024 us | avc_policy_seqno();
>> 2) | inode_has_perm.isra.21() {
>> 2) 0.148 us | avc_has_perm();
>> 2) 0.332 us | }
>> 2) 0.728 us | }
>> 2) 0.023 us | __fsnotify_parent();
>> 2) 0.034 us | fsnotify();
>> 2) 1.359 us | }
>> 2) 0.045 us | file_ra_state_init();
>> 2) 2.548 us | }
>> 2) 2.733 us | }
>> 2) 4.667 us | }
>> 2) | ima_file_check() {
>> 2) 0.028 us | process_measurement();
>> 2) 0.211 us | }
>> 2) 0.023 us | nfserrno [nfsd]();
>> 2) + 20.549 us | }
>>
>> And that’s 20us total for this nfsd_open. Seems like a lot of
>> work for each NFSv3 WRITE operation, and plenty of opportunities
>> for lock contention if my server happened to be otherwise busy.
>>
>> Note these figures are a little inflated because ftrace itself
>> adds some latency.
>>
>> Anyway, making fh_verify faster could have a significant
>> impact on per-op latency with fast storage. The recently-proposed
>> NFSD open cache by itself could eliminate the need to perform
>> fh_verify so often, for example.
>>
>
> Have you compared with Jeff's patchset? It would be very interesting
> to see how that affects your numbers.
Bruce asked me to post what I had so far, to help justify
adding the NFSD open cache, which has been tabled. Do you
know where Jeff keeps this work, and is it merged up to
4.4-rc ?
--
Chuck Lever
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