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Prevent TCP Cubic to abruptly increase cwnd after app-limited

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Cubic calculates the new cwnd based on absolute time
elapsed since the start of an epoch. A cubic epoch is
started on congestion events, or once the congestion
avoidance phase is started, after slow-start has
completed.

When a sender is application limited for an extended
amount of time and subsequently a larger volume of data
becomes ready for sending, Cubic recalculates cwnd
with a lingering cubic epoch. This recalculation
of the cwnd can induce a massive increase in cwnd,
causing a burst of data to be sent at line rate by
the sender.

This adds a flag to reset the cubic epoch once a
session transitions from app-limited to cwnd-limited
to prevent the above effect.

Reviewed by:	chengc_netapp.com, tuexen (mentor)
Approved by:	tuexen (mentor), rgrimes (mentor)
MFC after:	3 weeks
Sponsored by:	NetApp, Inc.
Differential Revision:	https://reviews.freebsd.org/D25065
This commit is contained in:
Richard Scheffenegger 2020-06-10 07:32:02 +00:00
parent 2b2c6d6919
commit 2fda0a6f3a
1 changed files with 8 additions and 2 deletions
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10
sys/netinet/cc/cc_cubic.c
View File

@ -94,6 +94,7 @@ struct cubic {
uint32_t flags;
#define CUBICFLAG_CONG_EVENT 0x00000001 /* congestion experienced */
#define CUBICFLAG_IN_SLOWSTART 0x00000002 /* in slow start */
#define CUBICFLAG_IN_APPLIMIT 0x00000004 /* application limited */
/* Minimum observed rtt in ticks. */
int min_rtt_ticks;
/* Mean observed rtt between congestion epochs. */
@ -153,8 +154,10 @@ cubic_ack_received(struct cc_var *ccv, uint16_t type)
cubic_data->t_last_cong = ticks - INT_MAX;
}
if (cubic_data->flags & CUBICFLAG_IN_SLOWSTART) {
cubic_data->flags &= ~CUBICFLAG_IN_SLOWSTART;
if (cubic_data->flags & (CUBICFLAG_IN_SLOWSTART |
CUBICFLAG_IN_APPLIMIT)) {
cubic_data->flags &= ~(CUBICFLAG_IN_SLOWSTART |
CUBICFLAG_IN_APPLIMIT);
cubic_data->t_last_cong = ticks;
cubic_data->K = 0;
}
@ -214,6 +217,9 @@ cubic_ack_received(struct cc_var *ccv, uint16_t type)
CCV(ccv, t_maxseg));
}
}
} else if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
!(ccv->flags & CCF_CWND_LIMITED)) {
cubic_data->flags |= CUBICFLAG_IN_APPLIMIT;
}
}