b4f70c3d4e
This patch enables tail loss probe in cwnd reduction (CWR) state to detect potential losses. Prior to this patch, since the sender uses PRR to determine the cwnd in CWR state, the combination of CWR+PRR plus tcp_tso_should_defer() could cause unnecessary stalls upon losses: PRR makes cwnd so gentle that tcp_tso_should_defer() defers sending wait for more ACKs. The ACKs may not come due to packet losses. Disallowing TLP when there is unused cwnd had the primary effect of disallowing TLP when there is TSO deferral, Nagle deferral, or we hit the rwin limit. Because basically every application write() or incoming ACK will cause us to run tcp_write_xmit() to see if we can send more, and then if we sent something we call tcp_schedule_loss_probe() to see if we should schedule a TLP. At that point, there are a few common reasons why some cwnd budget could still be unused: (a) rwin limit (b) nagle check (c) TSO deferral (d) TSQ For (d), after the next packet tx completion the TSQ mechanism will allow us to send more packets, so we don't really need a TLP (in practice it shouldn't matter whether we schedule one or not). But for (a), (b), (c) the sender won't send any more packets until it gets another ACK. But if the whole flight was lost, or all the ACKs were lost, then we won't get any more ACKs, and ideally we should schedule and send a TLP to get more feedback. In particular for a long time we have wanted some kind of timer for TSO deferral, and at least this would give us some kind of timer Reported-by: Steve Ibanez <sibanez@stanford.edu> Signed-off-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: Yuchung Cheng <ycheng@google.com> Reviewed-by: Nandita Dukkipati <nanditad@google.com> Reviewed-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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README
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.