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Date: Tue, 11 Oct 2016 18:01:41 -0700
From: Yuchung Cheng <ycheng@...gle.com>
To: Lawrence Brakmo <brakmo@...com>
Cc: netdev <netdev@...r.kernel.org>, Kernel Team <Kernel-team@...com>,
Eric Dumazet <eric.dumazet@...il.com>,
Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next] tcp: Change txhash on some non-RTO retransmits
On Tue, Oct 11, 2016 at 2:08 PM, Lawrence Brakmo <brakmo@...com> wrote:
> Yuchung, thank you for your comments. Responses inline.
>
> On 10/11/16, 12:49 PM, "Yuchung Cheng" <ycheng@...gle.com> wrote:
>
>>On Mon, Oct 10, 2016 at 5:18 PM, Lawrence Brakmo <brakmo@...com> wrote:
>>>
>>> The purpose of this patch is to help balance flows across paths. A new
>>> sysctl "tcp_retrans_txhash_prob" specifies the probability (0-100) that
>>> the txhash (IPv6 flowlabel) will be changed after a non-RTO retransmit.
>>> A probability is used in order to control how many flows are moved
>>> during a congestion event and prevent the congested path from becoming
>>> under utilized (which could occur if too many flows leave the current
>>> path). Txhash changes may be delayed in order to decrease the likelihood
>>> that it will trigger retransmists due to too much reordering.
>>>
>>> Another sysctl "tcp_retrans_txhash_mode" determines the behavior after
>>> RTOs. If the sysctl is 0, then after an RTO, only RTOs can trigger
>>> txhash changes. The idea is to decrease the likelihood of going back
>>> to a broken path. That is, we don't want flow balancing to trigger
>>> changes to broken paths. The drawback is that flow balancing does
>>> not work as well. If the sysctl is greater than 1, then we always
>>> do flow balancing, even after RTOs.
>>>
>>> Tested with packedrill tests (for correctness) and performance
>>> experiments with 2 and 3 paths. Performance experiments looked at
>>> aggregate goodput and fairness. For each run, we looked at the ratio of
>>> the goodputs for the fastest and slowest flows. These were averaged for
>>> all the runs. A fairness of 1 means all flows had the same goodput, a
>>> fairness of 2 means the fastest flow was twice as fast as the slowest
>>> flow.
>>>
>>> The setup for the performance experiments was 4 or 5 serves in a rack,
>>> 10G links. I tested various probabilities, but 20 seemed to have the
>>> best tradeoff for my setup (small RTTs).
>>>
>>> --- node1 -----
>>> sender --- switch --- node2 ----- switch ---- receiver
>>> --- node3 -----
>>>
>>> Scenario 1: One sender sends to one receiver through 2 routes (node1 or
>>> node 2). The output from node1 and node2 is 1G (1gbit/sec). With only 2
>>> flows, without flow balancing (prob=0) the average goodput is 1.6G vs.
>>> 1.9G with flow balancing due to 2 flows ending up in one link and either
>>> not moving and taking some time to move. Fairness was 1 in all cases.
>>> For 7 flows, goodput was 1.9G for all, but fairness was 1.5, 1.4 or 1.2
>>> for prob=0, prob=20,mode=0 and prob=20,mode=1 respectively. That is,
>>> flow balancing increased fairness.
>>>
>>> Scenario 2: One sender to one receiver, through 3 routes (node1,...
>>> node2). With 6 or 16 flows the goodput was the same for all, but
>>> fairness was 1.8, 1.5 and 1.2 respectively. Interestingly, the worst
>>> case fairness out of 10 runs were 2.2, 1.8 and 1.4 repectively. That is,
>>> prob=20,mode=1 improved average and worst case fairness.
>>I am wondering if we can build better API with routing layer to
>>implement this type of feature, instead of creeping the tx_rehashing
>>logic scatter in TCP. For example, we call dst_negative_advice on TCP
>>write timeouts.
>
> Not sure. The route is not necessarily bad, may be temporarily congested
> or they may all be congested. If all we want to do is change the txhash
> (unlike dst_negative_advice), then calling a tx_rehashing function may
> be the appropriate call.
>
>>
>>On the patch itself, it seems aggressive to (attempt to) rehash every
>>post-RTO retranmission. Also you can just use ca_state (==CA_Loss) to
>>identify post-RTO retransmission directly.
>
> Thanks, I will add the test.
>
>>
>>is this an implementation of the Flow Bender ?
>>https://urldefense.proofpoint.com/v2/url?u=http-3A__dl.acm.org_citation.cf
>>m-3Fid-3D2674985&d=DQIBaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=pq_Mqvzfy-C8ltkgyx1u_
>>g&m=Q4nONH7kQ5AvQguw9UxpcHd79jfdDdrXj1YSJs7Ezhk&s=MA4fWBLMTGgRS0eGvBjxf7BJ
>>Ol3-oxAzZDEYUG4cE-s&e=
>
> Part of flow bender, although there are also some similarities to flowlet
> switching.
>
>>
>>>
>>> Scenario 3: One sender to one receiver, 2 routes, one route drops 50% of
>>> the packets. With 7 flows, goodput was the same 1.1G, but fairness was
>>> 1.8, 2.0 and 2.1 respectively. That is, if there is a bad route, then
>>> balancing, which does more re-routes, is less fair.
>>>
>>> Signed-off-by: Lawrence Brakmo <brakmo@...com>
>>> ---
>>> Documentation/networking/ip-sysctl.txt | 15 +++++++++++++++
>>> include/linux/tcp.h | 4 +++-
>>> include/net/tcp.h | 2 ++
>>> net/ipv4/sysctl_net_ipv4.c | 18 ++++++++++++++++++
>>> net/ipv4/tcp_input.c | 10 ++++++++++
>>> net/ipv4/tcp_output.c | 23 ++++++++++++++++++++++-
>>> net/ipv4/tcp_timer.c | 4 ++++
>>> 7 files changed, 74 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/Documentation/networking/ip-sysctl.txt
>>>b/Documentation/networking/ip-sysctl.txt
>>> index 3db8c67..87a984c 100644
>>> --- a/Documentation/networking/ip-sysctl.txt
>>> +++ b/Documentation/networking/ip-sysctl.txt
>>> @@ -472,6 +472,21 @@ tcp_max_reordering - INTEGER
>>> if paths are using per packet load balancing (like bonding rr
>>>mode)
>>> Default: 300
>>>
>>> +tcp_retrans_txhash_mode - INTEGER
>>> + If zero, disable txhash recalculation due to non-RTO
>>>retransmissions
>>> + after an RTO. The idea is that broken paths will trigger an RTO
>>>and
>>> + we don't want going back to that path due to standard
>>>retransmissons
>>> + (flow balancing). The drawback is that balancing is less robust.
>>> + If greater than zero, can always (probabilistically) recalculate
>>> + txhash after non-RTO retransmissions.
>>> +
>>> +tcp_retrans_txhash_prob - INTEGER
>>> + Probability [0 to 100] that we will recalculate txhash when a
>>> + packet is resent not due to RTO (for RTO txhash is always
>>>recalculated).
>>> + The recalculation of the txhash may be delayed to decrease the
>>> + likelihood that reordering will trigger retransmissons.
>>> + The purpose is to help balance the flows among the possible
>>>paths.
>>> +
>>> tcp_retrans_collapse - BOOLEAN
>>> Bug-to-bug compatibility with some broken printers.
>>> On retransmit try to send bigger packets to work around bugs in
>>> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
>>> index a17ae7b..e0e3b7d 100644
>>> --- a/include/linux/tcp.h
>>> +++ b/include/linux/tcp.h
>>> @@ -214,7 +214,9 @@ struct tcp_sock {
>>> } rack;
>>> u16 advmss; /* Advertised MSS
>>>*/
>>> u8 rate_app_limited:1, /* rate_{delivered,interval_us}
>>>limited? */
>>> - unused:7;
>>> + txhash_rto:1, /* If set, don't do flow balancing
>>>*/
>>> + txhash_want:1, /* We want to change txhash when safe
>>>*/
>>> + unused:5;
>>> u8 nonagle : 4,/* Disable Nagle algorithm?
>>>*/
>>> thin_lto : 1,/* Use linear timeouts for thin streams
>>>*/
>>> thin_dupack : 1,/* Fast retransmit on first dupack
>>>*/
>>> diff --git a/include/net/tcp.h b/include/net/tcp.h
>>> index f83b7f2..3abd304 100644
>>> --- a/include/net/tcp.h
>>> +++ b/include/net/tcp.h
>>> @@ -271,6 +271,8 @@ extern int sysctl_tcp_autocorking;
>>> extern int sysctl_tcp_invalid_ratelimit;
>>> extern int sysctl_tcp_pacing_ss_ratio;
>>> extern int sysctl_tcp_pacing_ca_ratio;
>>> +extern int sysctl_tcp_retrans_txhash_prob;
>>> +extern int sysctl_tcp_retrans_txhash_mode;
>>>
>>> extern atomic_long_t tcp_memory_allocated;
>>> extern struct percpu_counter tcp_sockets_allocated;
>>> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
>>> index 1cb67de..00d6f26 100644
>>> --- a/net/ipv4/sysctl_net_ipv4.c
>>> +++ b/net/ipv4/sysctl_net_ipv4.c
>>> @@ -28,6 +28,7 @@
>>> static int zero;
>>> static int one = 1;
>>> static int four = 4;
>>> +static int hundred = 100;
>>> static int thousand = 1000;
>>> static int gso_max_segs = GSO_MAX_SEGS;
>>> static int tcp_retr1_max = 255;
>>> @@ -624,6 +625,23 @@ static struct ctl_table ipv4_table[] = {
>>> .proc_handler = proc_dointvec_ms_jiffies,
>>> },
>>> {
>>> + .procname = "tcp_retrans_txhash_prob",
>>> + .data = &sysctl_tcp_retrans_txhash_prob,
>>> + .maxlen = sizeof(int),
>>> + .mode = 0644,
>>> + .proc_handler = proc_dointvec_minmax,
>>> + .extra1 = &zero,
>>> + .extra2 = &hundred,
>>> + },
>>> + {
>>> + .procname = "tcp_retrans_txhash_mode",
>>> + .data = &sysctl_tcp_retrans_txhash_mode,
>>> + .maxlen = sizeof(int),
>>> + .mode = 0644,
>>> + .proc_handler = proc_dointvec_minmax,
>>> + .extra1 = &zero,
>>> + },
>>> + {
>>> .procname = "icmp_msgs_per_sec",
>>> .data = &sysctl_icmp_msgs_per_sec,
>>> .maxlen = sizeof(int),
>>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>>> index a27b9c0..fed5366 100644
>>> --- a/net/ipv4/tcp_input.c
>>> +++ b/net/ipv4/tcp_input.c
>>> @@ -101,6 +101,9 @@ int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
>>> int sysctl_tcp_early_retrans __read_mostly = 3;
>>> int sysctl_tcp_invalid_ratelimit __read_mostly = HZ/2;
>>>
>>> +int sysctl_tcp_retrans_txhash_prob __read_mostly;
>>> +int sysctl_tcp_retrans_txhash_mode __read_mostly;
>>> +
>>> #define FLAG_DATA 0x01 /* Incoming frame contained data.
>>> */
>>> #define FLAG_WIN_UPDATE 0x02 /* Incoming ACK was a
>>>window update. */
>>> #define FLAG_DATA_ACKED 0x04 /* This ACK acknowledged
>>>new data. */
>>> @@ -3674,6 +3677,13 @@ static int tcp_ack(struct sock *sk, const struct
>>>sk_buff *skb, int flag)
>>> flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una,
>>>&acked,
>>> &sack_state, &now);
>>>
>>> + /* Check if we should set txhash (would not cause reordering) */
>>> + if (tp->txhash_want &&
>>> + (tp->packets_out - tp->sacked_out) < tp->reordering) {
>>> + sk_set_txhash(sk);
>>> + tp->txhash_want = 0;
>>> + }
>>> +
>>> if (tcp_ack_is_dubious(sk, flag)) {
>>> is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED |
>>>FLAG_NOT_DUP));
>>> tcp_fastretrans_alert(sk, acked, is_dupack, &flag,
>>>&rexmit);
>>> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
>>> index 896e9df..58490ac 100644
>>> --- a/net/ipv4/tcp_output.c
>>> +++ b/net/ipv4/tcp_output.c
>>> @@ -2738,9 +2738,30 @@ int tcp_retransmit_skb(struct sock *sk, struct
>>>sk_buff *skb, int segs)
>>> tp->retrans_out += tcp_skb_pcount(skb);
>>>
>>> /* Save stamp of the first retransmit. */
>>> - if (!tp->retrans_stamp)
>>> + if (!tp->retrans_stamp) {
>>> tp->retrans_stamp = tcp_skb_timestamp(skb);
>>>
>>> + /* Determine if we should reset hash, only done
>>>once
>>> + * per recovery
>>> + */
>>> + if ((!tp->txhash_rto ||
>>> + sysctl_tcp_retrans_txhash_mode > 0) &&
>>> + sk->sk_txhash &&
>>> + (prandom_u32_max(100) <
>>> + sysctl_tcp_retrans_txhash_prob)) {
>>> + /* If not too much reordering, or RTT is
>>> + * small enough that we don't care about
>>> + * reordering, then change it now.
>>> + * Else, wait until it is safe.
>>> + */
>>> + if ((tp->packets_out - tp->sacked_out) <
>>> + tp->reordering)
>>I don't parse this logic ... suppose reordering is 100 (not uncommon
>>today due to the last packet being delivered slightly earlier than the
>>rest), and cwnd==packets_out =~200,we only want to rehash until half
>>of the packets are sacked, so we are still rehashing even when
>>reordering is heavy?
>
> In your scenario, there would be no re-hashing until sacked_out is 101
> ((packets_out - sacked_out) < 100). This code would mark txhash_want.
> Then when an ACK is received and the conditional is true, txhash
> would be changed.
>
> Now, the test does not prevent retransmissions due to reordering in all
> cases, but hopefully in most. I will also add the test you recommended,
> checking for CA_Loss, to prevent too much re-hashing.
If the whole point is to rehash at most once between recovery events,
why do we need this complicated change at per packet level in
tcp_retransmit_skb()? there are functions that start and end recovery
(tcp_enter_recovery, tcp_end_cwnd_reduction). our retransmission logic
is already very complicated.
I still don't understand the incentive of starting the rehashing
half-way retransmitting depending on the sacking and reordering
status, for temporarily congestion as you've mentioned.
is this feature unique for intra-DC connections?
>
>>
>>also where do we check RTT is small?
>
> The RTT comment is left over from a previous version, I will remove it.
>
>>
>>> + sk_set_txhash(sk);
>>> + else
>>> + tp->txhash_want = 1;
>>> + }
>>> + }
>>> +
>>> } else if (err != -EBUSY) {
>>> NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL);
>>> }
>>> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
>>> index 3ea1cf8..e66baad 100644
>>> --- a/net/ipv4/tcp_timer.c
>>> +++ b/net/ipv4/tcp_timer.c
>>> @@ -186,6 +186,8 @@ static int tcp_write_timeout(struct sock *sk)
>>>
>>> if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
>>> if (icsk->icsk_retransmits) {
>>> + tp->txhash_rto = 1;
>>> + tp->txhash_want = 0;
>>> dst_negative_advice(sk);
>>> if (tp->syn_fastopen || tp->syn_data)
>>> tcp_fastopen_cache_set(sk, 0, NULL,
>>>true, 0);
>>> @@ -218,6 +220,8 @@ static int tcp_write_timeout(struct sock *sk)
>>> } else {
>>> sk_rethink_txhash(sk);
>>> }
>>> + tp->txhash_rto = 1;
>>> + tp->txhash_want = 0;
>>>
>>> retry_until = net->ipv4.sysctl_tcp_retries2;
>>> if (sock_flag(sk, SOCK_DEAD)) {
>>> --
>>> 2.9.3
>>>
>
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