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| -rw-r--r-- | doc/design-thoughts/connection-reuse.txt | 224 |
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diff --git a/doc/design-thoughts/connection-reuse.txt b/doc/design-thoughts/connection-reuse.txt new file mode 100644 index 0000000..4eb22f6 --- /dev/null +++ b/doc/design-thoughts/connection-reuse.txt @@ -0,0 +1,224 @@ +2015/08/06 - server connection sharing + +Improvements on the connection sharing strategies +------------------------------------------------- + +4 strategies are currently supported : + - never + - safe + - aggressive + - always + +The "aggressive" and "always" strategies take into account the fact that the +connection has already been reused at least once or not. The principle is that +second requests can be used to safely "validate" connection reuse on newly +added connections, and that such validated connections may be used even by +first requests from other sessions. A validated connection is a connection +which has already been reused, hence proving that it definitely supports +multiple requests. Such connections are easy to verify : after processing the +response, if the txn already had the TX_NOT_FIRST flag, then it was not the +first request over that connection, and it is validated as safe for reuse. +Validated connections are put into a distinct list : server->safe_conns. + +Incoming requests with TX_NOT_FIRST first pick from the regular idle_conns +list so that any new idle connection is validated as soon as possible. + +Incoming requests without TX_NOT_FIRST only pick from the safe_conns list for +strategy "aggressive", guaranteeing that the server properly supports connection +reuse, or first from the safe_conns list, then from the idle_conns list for +strategy "always". + +Connections are always stacked into the list (LIFO) so that there are higher +changes to convert recent connections and to use them. This will first optimize +the likeliness that the connection works, and will avoid TCP metrics from being +lost due to an idle state, and/or the congestion window to drop and the +connection going to slow start mode. + + +Handling connections in pools +----------------------------- + +A per-server "pool-max" setting should be added to permit disposing unused idle +connections not attached anymore to a session for use by future requests. The +principle will be that attached connections are queued from the front of the +list while the detached connections will be queued from the tail of the list. + +This way, most reused connections will be fairly recent and detached connections +will most often be ignored. The number of detached idle connections in the lists +should be accounted for (pool_used) and limited (pool_max). + +After some time, a part of these detached idle connections should be killed. +For this, the list is walked from tail to head and connections without an owner +may be evicted. It may be useful to have a per-server pool_min setting +indicating how many idle connections should remain in the pool, ready for use +by new requests. Conversely, a pool_low metric should be kept between eviction +runs, to indicate the lowest amount of detached connections that were found in +the pool. + +For eviction, the principle of a half-life is appealing. The principle is +simple : over a period of time, half of the connections between pool_min and +pool_low should be gone. Since pool_low indicates how many connections were +remaining unused over a period, it makes sense to kill some of them. + +In order to avoid killing thousands of connections in one run, the purge +interval should be split into smaller batches. Let's call N the ratio of the +half-life interval and the effective interval. + +The algorithm consists in walking over them from the end every interval and +killing ((pool_low - pool_min) + 2 * N - 1) / (2 * N). It ensures that half +of the unused connections are killed over the half-life period, in N batches +of population/2N entries at most. + +Unsafe connections should be evicted first. There should be quite few of them +since most of them are probed and become safe. Since detached connections are +quickly recycled and attached to a new session, there should not be too many +detached connections in the pool, and those present there may be killed really +quickly. + +Another interesting point of pools is that when a pool-max is not null, then it +makes sense to automatically enable pretend-keep-alive on non-private connections +going to the server in order to be able to feed them back into the pool. With +the "aggressive" or "always" strategies, it can allow clients making a single +request over their connection to share persistent connections to the servers. + + + +2013/10/17 - server connection management and reuse + +Current state +------------- + +At the moment, a connection entity is needed to carry any address +information. This means in the following situations, we need a server +connection : + +- server is elected and the server's destination address is set + +- transparent mode is elected and the destination address is set from + the incoming connection + +- proxy mode is enabled, and the destination's address is set during + the parsing of the HTTP request + +- connection to the server fails and must be retried on the same + server using the same parameters, especially the destination + address (SN_ADDR_SET not removed) + + +On the accepting side, we have further requirements : + +- allocate a clean connection without a stream interface + +- incrementally set the accepted connection's parameters without + clearing it, and keep track of what is set (eg: getsockname). + +- initialize a stream interface in established mode + +- attach the accepted connection to a stream interface + + +This means several things : + +- the connection has to be allocated on the fly the first time it is + needed to store the source or destination address ; + +- the connection has to be attached to the stream interface at this + moment ; + +- it must be possible to incrementally set some settings on the + connection's addresses regardless of the connection's current state + +- the connection must not be released across connection retries ; + +- it must be possible to clear a connection's parameters for a + redispatch without having to detach/attach the connection ; + +- we need to allocate a connection without an existing stream interface + +So on the accept() side, it looks like this : + + fd = accept(); + conn = new_conn(); + get_some_addr_info(&conn->addr); + ... + si = new_si(); + si_attach_conn(si, conn); + si_set_state(si, SI_ST_EST); + ... + get_more_addr_info(&conn->addr); + +On the connect() side, it looks like this : + + si = new_si(); + while (!properly_connected) { + if (!(conn = si->end)) { + conn = new_conn(); + conn_clear(conn); + si_attach_conn(si, conn); + } + else { + if (connected) { + f = conn->flags & CO_FL_XPRT_TRACKED; + conn->flags &= ~CO_FL_XPRT_TRACKED; + conn_close(conn); + conn->flags |= f; + } + if (!correct_dest) + conn_clear(conn); + } + set_some_addr_info(&conn->addr); + si_set_state(si, SI_ST_CON); + ... + set_more_addr_info(&conn->addr); + conn->connect(); + if (must_retry) { + close_conn(conn); + } + } + +Note: we need to be able to set the control and transport protocols. +On outgoing connections, this is set once we know the destination address. +On incoming connections, this is set the earliest possible (once we know +the source address). + +The problem analysed below was solved on 2013/10/22 + +| ==> the real requirement is to know whether a connection is still valid or not +| before deciding to close it. CO_FL_CONNECTED could be enough, though it +| will not indicate connections that are still waiting for a connect to occur. +| This combined with CO_FL_WAIT_L4_CONN and CO_FL_WAIT_L6_CONN should be OK. +| +| Alternatively, conn->xprt could be used for this, but needs some careful checks +| (it's used by conn_full_close at least). +| +| Right now, conn_xprt_close() checks conn->xprt and sets it to NULL. +| conn_full_close() also checks conn->xprt and sets it to NULL, except +| that the check on ctrl is performed within xprt. So conn_xprt_close() +| followed by conn_full_close() will not close the file descriptor. +| Note that conn_xprt_close() is never called, maybe we should kill it ? +| +| Note: at the moment, it's problematic to leave conn->xprt to NULL before doing +| xprt_init() because we might end up with a pending file descriptor. Or at +| least with some transport not de-initialized. We might thus need +| conn_xprt_close() when conn_xprt_init() fails. +| +| The fd should be conditioned by ->ctrl only, and the transport layer by ->xprt. +| +| - conn_prepare_ctrl(conn, ctrl) +| - conn_prepare_xprt(conn, xprt) +| - conn_prepare_data(conn, data) +| +| Note: conn_xprt_init() needs conn->xprt so it's not a problem to set it early. +| +| One problem might be with conn_xprt_close() not being able to know if xprt_init() +| was called or not. That's where it might make sense to only set ->xprt during init. +| Except that it does not fly with outgoing connections (xprt_init is called after +| connect()). +| +| => currently conn_xprt_close() is only used by ssl_sock.c and decides whether +| to do something based on ->xprt_ctx which is set by ->init() from xprt_init(). +| So there is nothing to worry about. We just need to restore conn_xprt_close() +| and rely on ->ctrl to close the fd instead of ->xprt. +| +| => we have the same issue with conn_ctrl_close() : when is the fd supposed to be +| valid ? On outgoing connections, the control is set much before the fd... |