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

From: Stephen Hemminger <[email-masked]>
Date: Tue, 29 Sep 2009 08:45:34 -0700

On Tue, 29 Sep 2009 11:55:18 +0300
Yakov Lerner <[email-masked]> wrote:

> On Tue, Sep 29, 2009 at 10:56, Eric Dumazet <[email-masked]> 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 <[email-masked]>
> > > ---
> > >  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.

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