Re: [PATCH] /proc/net/tcp, overhead removed

[Yakov Lerner <iler.ml@gmail.com>]

On Tue, Sep 29, 2009 at 18:45, Stephen Hemminger <shemminger@vyatta.com> wrote:
> On Tue, 29 Sep 2009 11:55:18 +0300
> Yakov Lerner <iler.ml@gmail.com> wrote:
>
>> On Tue, Sep 29, 2009 at 10:56, Eric Dumazet <eric.dumazet@gmail.com> wrote:
>> >
>> > Yakov Lerner a écrit :
>> > > Take 2.
>> > >
>> > > "Sharp improvement in performance of /proc/net/tcp when number of
>> > > sockets is large and hashsize is large.
>> > > O(numsock * hashsize) time becomes O(numsock + hashsize). On slow
>> > > processors, speed difference can be x100 and more."
>> > >
>> > > I must say that I'm not fully satisfied with my choice of "st->sbucket"
>> > > for the new preserved index. The better name would be "st->snum".
>> > > Re-using "st->sbucket" saves 4 bytes, and keeps the patch to one sourcefile.
>> > > But "st->sbucket" has different meaning in OPENREQ and LISTEN states;
>> > > this can be confusing.
>> > > Maybe better add "snum" member to struct tcp_iter_state ?
>> > >
>> > > Shall I change subject when sending "take N+1", or keep the old subject ?
>> > >
>> > > Signed-off-by: Yakov Lerner <iler.ml@gmail.com>
>> > > ---
>> > >  net/ipv4/tcp_ipv4.c |   35 +++++++++++++++++++++++++++++++++--
>> > >  1 files changed, 33 insertions(+), 2 deletions(-)
>> > >
>> > > diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
>> > > index 7cda24b..e4c4f19 100644
>> > > --- a/net/ipv4/tcp_ipv4.c
>> > > +++ b/net/ipv4/tcp_ipv4.c
>> > > @@ -1994,13 +1994,14 @@ static inline int empty_bucket(struct tcp_iter_state *st)
>> > >               hlist_nulls_empty(&tcp_hashinfo.ehash[st->bucket].twchain);
>> > >  }
>> > >
>> > > -static void *established_get_first(struct seq_file *seq)
>> > > +static void *established_get_first_after(struct seq_file *seq, int bucket)
>> > >  {
>> > >       struct tcp_iter_state *st = seq->private;
>> > >       struct net *net = seq_file_net(seq);
>> > >       void *rc = NULL;
>> > >
>> > > -     for (st->bucket = 0; st->bucket < tcp_hashinfo.ehash_size; ++st->bucket) {
>> > > +     for (st->bucket = bucket; st->bucket < tcp_hashinfo.ehash_size;
>> > > +          ++st->bucket) {
>> > >               struct sock *sk;
>> > >               struct hlist_nulls_node *node;
>> > >               struct inet_timewait_sock *tw;
>> > > @@ -2010,6 +2011,8 @@ static void *established_get_first(struct seq_file *seq)
>> > >               if (empty_bucket(st))
>> > >                       continue;
>> > >
>> > > +             st->sbucket = st->num;
>> > > +
>> > >               spin_lock_bh(lock);
>> > >               sk_nulls_for_each(sk, node, &tcp_hashinfo.ehash[st->bucket].chain) {
>> > >                       if (sk->sk_family != st->family ||
>> > > @@ -2036,6 +2039,11 @@ out:
>> > >       return rc;
>> > >  }
>> > >
>> > > +static void *established_get_first(struct seq_file *seq)
>> > > +{
>> > > +     return established_get_first_after(seq, 0);
>> > > +}
>> > > +
>> > >  static void *established_get_next(struct seq_file *seq, void *cur)
>> > >  {
>> > >       struct sock *sk = cur;
>> > > @@ -2064,6 +2072,9 @@ get_tw:
>> > >               while (++st->bucket < tcp_hashinfo.ehash_size &&
>> > >                               empty_bucket(st))
>> > >                       ;
>> > > +
>> > > +             st->sbucket = st->num;
>> > > +
>> > >               if (st->bucket >= tcp_hashinfo.ehash_size)
>> > >                       return NULL;
>> > >
>> > > @@ -2107,6 +2118,7 @@ static void *tcp_get_idx(struct seq_file *seq, loff_t pos)
>> > >
>> > >       if (!rc) {
>> > >               st->state = TCP_SEQ_STATE_ESTABLISHED;
>> > > +             st->sbucket = 0;
>> > >               rc        = established_get_idx(seq, pos);
>> > >       }
>> > >
>> > > @@ -2116,6 +2128,25 @@ static void *tcp_get_idx(struct seq_file *seq, loff_t pos)
>> > >  static void *tcp_seq_start(struct seq_file *seq, loff_t *pos)
>> > >  {
>> > >       struct tcp_iter_state *st = seq->private;
>> > > +
>> > > +     if (*pos && *pos >= st->sbucket &&
>> > > +         (st->state == TCP_SEQ_STATE_ESTABLISHED ||
>> > > +          st->state == TCP_SEQ_STATE_TIME_WAIT)) {
>> > > +             void *cur;
>> > > +             int nskip;
>> > > +
>> > > +             /* for states estab and tw, st->sbucket is index (*pos) */
>> > > +             /* corresponding to the beginning of bucket st->bucket */
>> > > +
>> > > +             st->num = st->sbucket;
>> > > +             /* jump to st->bucket, then skip (*pos - st->sbucket) items */
>> > > +             st->state = TCP_SEQ_STATE_ESTABLISHED;
>> > > +             cur = established_get_first_after(seq, st->bucket);
>> > > +             for (nskip = *pos - st->num; cur && nskip > 0; --nskip)
>> > > +                     cur = established_get_next(seq, cur);
>> > > +             return cur;
>> > > +     }
>> > > +
>> > >       st->state = TCP_SEQ_STATE_LISTENING;
>> > >       st->num = 0;
>> > >       return *pos ? tcp_get_idx(seq, *pos - 1) : SEQ_START_TOKEN;
>> >
>> > Just in case you are working on "take 3" of the patch, there is a fondamental problem.
>> >
>> > All the scalability problems come from the fact that tcp_seq_start()
>> > *has* to rescan all the tables from the begining, because of lseek() capability
>> > on /proc/net/tcp file
>> >
>> > We probably could disable llseek() (on other positions than start of the file),
>> > and rely only on internal state (listening/established hashtable, hash bucket, position in chain)
>> >
>> > I cannot imagine how an application could rely on lseek() on >0 position in this file.
>>
>>
>> I thought  /proc/net/tcp  can  both  be fast and allow lseek;
>> (1) when no lseek was issued since last read
>> (we can detect this), /proc/net/tcp can jump to the
>> last known bucket (common case), vs
>> (2) switch to slow mode (scan from the beginning of hash)
>> when lseek was used , no ?
>
> If you look at fib_hash and fib_trie, they already do the same thing.
>  * fib_hash records last hash chain to avoid overhead of rescan.
>  * fib_trie records last route and does fast lookup to restart from there.

Thanks for the pointer.