[ipxe-devel] TCP flow control window shrink on discard causes low throughput

[Steven Haber]

No love? We've been sitting with this patch on top of our iPXE source. It's
pretty heinous.

On Thu, Dec 4, 2014 at 4:02 PM, Steven Haber <steven at qumulo.com> wrote:

> Hello!
>
> As the subject states, I'm seeing a situation where the TCP window
> shrink logic causes a connection to be throttled indefinitely. I am
> downloading a ~600MB ISO image from an internal server using iPXE's
> HTTP capabilities. Intermittently I will see the download slow to a
> crawl. A packet capture shows the receive side decreasing the TCP
> window drastically and permanently, from a reasonable size of ~150k
> down to 1k. I can send the capture to you if it'd be useful. The
> responsible code lives in src/net/tcp.c:tcp_discard. Below is a patch
> that remedies the issue. Obviously this is hack. The correct solution
> might involve capping the maximum window size decrease per discard,
> and adding complimentary window size increase code to the main rx
> path. This way slowdowns wouldn't be as severe and we could recover
> from them.
>
> What do you think? Thanks!
>
> Steven Haber
> Software Engineer
> Qumulo, Inc.
>
>
>
>
>
> diff --git a/src/net/tcp.c b/src/net/tcp.c
> index 987cb63..3c59e46 100644
> --- a/src/net/tcp.c
> +++ b/src/net/tcp.c
> @@ -1331,31 +1331,31 @@ static unsigned int tcp_discard ( void ) {
>         struct tcp_rx_queued_header *tcpqhdr;
>         uint32_t max_win;
>         unsigned int discarded = 0;
>
>         /* Try to drop one queued RX packet from each connection */
>         list_for_each_entry ( tcp, &tcp_conns, list ) {
>                 list_for_each_entry_reverse ( iobuf, &tcp->rx_queue, list
> ) {
>
>                         /* Limit window to prevent future discards */
>                         tcpqhdr = iobuf->data;
>                         max_win = ( tcpqhdr->seq - tcp->rcv_ack );
>                         if ( max_win < tcp->max_rcv_win ) {
>                                 DBGC ( tcp, "TCP %p reducing maximum
> window "
>                                        "from %d to %d\n",
>                                        tcp, tcp->max_rcv_win, max_win );
> -                               tcp->max_rcv_win = max_win;
> +                               //tcp->max_rcv_win = max_win;
>                         }
>
>                         /* Remove packet from queue */
>                         list_del ( &iobuf->list );
>                         free_iob ( iobuf );
>
>                         /* Report discard */
>                         discarded++;
>                         break;
>                 }
>         }
>
>         return discarded;
>  }
>
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