1. 25 Nov, 2008 7 commits
    • Ilpo Järvinen's avatar
      tcp: Try to restore large SKBs while SACK processing · 832d11c5
      Ilpo Järvinen authored
      During SACK processing, most of the benefits of TSO are eaten by
      the SACK blocks that one-by-one fragment SKBs to MSS sized chunks.
      Then we're in problems when cleanup work for them has to be done
      when a large cumulative ACK comes. Try to return back to pre-split
      state already while more and more SACK info gets discovered by
      combining newly discovered SACK areas with the previous skb if
      that's SACKed as well.
      This approach has a number of benefits:
      1) The processing overhead is spread more equally over the RTT
      2) Write queue has less skbs to process (affect everything
         which has to walk in the queue past the sacked areas)
      3) Write queue is consistent whole the time, so no other parts
         of TCP has to be aware of this (this was not the case with
         some other approach that was, well, quite intrusive all
      4) Clean_rtx_queue can release most of the pages using single
         put_page instead of previous PAGE_SIZE/mss+1 calls
      In case a hole is fully filled by the new SACK block, we attempt
      to combine the next skb too which allows construction of skbs
      that are even larger than what tso split them to and it handles
      hole per on every nth patterns that often occur during slow start
      overshoot pretty nicely. Though this to be really useful also
      a retransmission would have to get lost since cumulative ACKs
      advance one hole at a time in the most typical case.
      TODO: handle upwards only merging. That should be rather easy
      when segment is fully sacked but I'm leaving that as future
      work item (it won't make very large difference anyway since
      this current approach already covers quite a lot of normal
      I was earlier thinking of some sophisticated way of tracking
      timestamps of the first and the last segment but later on
      realized that it won't be that necessary at all to store the
      timestamp of the last segment. The cases that can occur are
      basically either:
        1) ambiguous => no sensible measurement can be taken anyway
        2) non-ambiguous is due to reordering => having the timestamp
           of the last segment there is just skewing things more off
           than does some good since the ack got triggered by one of
           the holes (besides some substle issues that would make
           determining right hole/skb even harder problem). Anyway,
           it has nothing to do with this change then.
      I choose to route some abnormal looking cases with goto noop,
      some could be handled differently (eg., by stopping the
      walking at that skb but again). In general, they either
      shouldn't happen at all or are rare enough to make no difference
      in practice.
      In theory this change (as whole) could cause some macroscale
      regression (global) because of cache misses that are taken over
      the round-trip time but it gets very likely better because of much
      less (local) cache misses per other write queue walkers and the
      big recovery clearing cumulative ack.
      Worth to note that these benefits would be very easy to get also
      without TSO/GSO being on as long as the data is in pages so that
      we can merge them. Currently I won't let that happen because
      DSACK splitting at fragment that would mess up pcounts due to
      sk_can_gso in tcp_set_skb_tso_segs. Once DSACKs fragments gets
      avoided, we have some conditions that can be made less strict.
      TODO: I will probably have to convert the excessive pointer
      passing to struct sacktag_state... :-)
      My testing revealed that considerable amount of skbs couldn't
      be shifted because they were cloned (most likely still awaiting
      tx reclaim)...
      [The rest is considering future work instead since I got
      repeatably EFAULT to tcpdump's recvfrom when I added
      pskb_expand_head to deal with clones, so I separated that
      into another, later patch]
      ...To counter that, I gave up on the fifth advantage:
      5) When growing previous SACK block, less allocs for new skbs
         are done, basically a new alloc is needed only when new hole
         is detected and when the previous skb runs out of frags space
      ...which now only happens of if reclaim is fast enough to dispose
      the clone before the SACK block comes in (the window is RTT long),
      otherwise we'll have to alloc some.
      With clones being handled I got these numbers (will be somewhat
      worse without that), taken with fine-grained mibs:
                        TCPSackShifted 398
                         TCPSackMerged 877
                  TCPSackShiftFallback 320
                   TCPSACKCOLLAPSEHOLE 12
      Signed-off-by: default avatarIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    • Ilpo Järvinen's avatar
      tcp: make tcp_sacktag_one able to handle partial skb too · f58b22fd
      Ilpo Järvinen authored
      This is preparatory work for SACK combiner patch which may
      have to count TCP state changes for only a part of the skb
      because it will intentionally avoids splitting skb to SACKed
      and not sacked parts.
      Signed-off-by: default avatarIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    • Ilpo Järvinen's avatar
      tcp: Make SACK code to split only at mss boundaries · adb92db8
      Ilpo Järvinen authored
      Sadly enough, this adds possible divide though we try to avoid
      it by checking one mss as common case.
      Signed-off-by: default avatarIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    • Ilpo Järvinen's avatar
      tcp: more aggressive skipping · e8bae275
      Ilpo Järvinen authored
      I knew already when rewriting the sacktag that this condition
      was too conservative, change it now since it prevent lot of
      useless work (especially in the sack shifter decision code
      that is being added by a later patch). This shouldn't change
      anything really, just save some processing regardless of the
      Signed-off-by: default avatarIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    • Ilpo Järvinen's avatar
    • Ilpo Järvinen's avatar
      tcp: collapse more than two on retransmission · 4a17fc3a
      Ilpo Järvinen authored
      I always had thought that collapsing up to two at a time was
      intentional decision to avoid excessive processing if 1 byte
      sized skbs are to be combined for a full mtu, and consecutive
      retransmissions would make the size of the retransmittee
      double each round anyway, but some recent discussion made me
      to understand that was not the case. Thus make collapse work
      more and wait less.
      It would be possible to take advantage of the shifting
      machinery (added in the later patch) in the case of paged
      data but that can be implemented on top of this change.
      tcp_skb_is_last check is now provided by the loop.
      I tested a bit (ss-after-idle-off, fill 4096x4096B xfer,
      10s sleep + 4096 x 1byte writes while dropping them for
      some a while with netem):
      . 16774097:16775545(1448) ack 1 win 46
      . 16775545:16776993(1448) ack 1 win 46
      . ack 16759617 win 2399
      P 16776993:16777217(224) ack 1 win 46
      . ack 16762513 win 2399
      . ack 16765409 win 2399
      . ack 16768305 win 2399
      . ack 16771201 win 2399
      . ack 16774097 win 2399
      . ack 16776993 win 2399
      . ack 16777217 win 2399
      P 16777217:16777257(40) ack 1 win 46
      . ack 16777257 win 2399
      P 16777257:16778705(1448) ack 1 win 46
      P 16778705:16780153(1448) ack 1 win 46
      FP 16780153:16781313(1160) ack 1 win 46
      . ack 16778705 win 2399
      . ack 16780153 win 2399
      F 1:1(0) ack 16781314 win 2399
      While without drop-all period I get this:
      . 16773585:16775033(1448) ack 1 win 46
      . ack 16764897 win 9367
      . ack 16767793 win 9367
      . ack 16770689 win 9367
      . ack 16773585 win 9367
      . 16775033:16776481(1448) ack 1 win 46
      P 16776481:16777217(736) ack 1 win 46
      . ack 16776481 win 9367
      . ack 16777217 win 9367
      P 16777217:16777218(1) ack 1 win 46
      P 16777218:16777219(1) ack 1 win 46
      P 16777219:16777220(1) ack 1 win 46
      P 16777247:16777248(1) ack 1 win 46
      . ack 16777218 win 9367
      . ack 16777219 win 9367
      . ack 16777233 win 9367
      . ack 16777248 win 9367
      P 16777248:16778696(1448) ack 1 win 46
      P 16778696:16780144(1448) ack 1 win 46
      FP 16780144:16781313(1169) ack 1 win 46
      . ack 16780144 win 9367
      F 1:1(0) ack 16781314 win 9367
      The window seems to be 30-40 segments, which were successfully
      combined into: P 16777217:16777257(40) ack 1 win 46
      Signed-off-by: default avatarIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    • Eric Dumazet's avatar
      net: avoid a pair of dst_hold()/dst_release() in ip_push_pending_frames() · a21bba94
      Eric Dumazet authored
      We can reduce pressure on dst entry refcount that slowdown UDP transmit
      path on SMP machines. This pressure is visible on RTP servers when
      delivering content to mediagateways, especially big ones, handling
      thousand of streams. Several cpus send UDP frames to the same
      destination, hence use the same dst entry.
      This patch makes ip_push_pending_frames() steal the refcount its
      callers had to take when filling inet->cork.dst.
      This doesnt avoid all refcounting, but still gives speedups on SMP,
      on UDP/RAW transmit path.
      Signed-off-by: default avatarEric Dumazet <dada1@cosmosbay.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
  2. 24 Nov, 2008 21 commits
  3. 23 Nov, 2008 4 commits
  4. 22 Nov, 2008 8 commits