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diff --git a/doc/internals/notes-layers.txt b/doc/internals/notes-layers.txt new file mode 100644 index 0000000..541c125 --- /dev/null +++ b/doc/internals/notes-layers.txt @@ -0,0 +1,330 @@ +2018-02-21 - Layering in haproxy 1.9 +------------------------------------ + +2 main zones : + - application : reads from conn_streams, writes to conn_streams, often uses + streams + + - connection : receives data from the network, presented into buffers + available via conn_streams, sends data to the network + + +The connection zone contains multiple layers which behave independently in each +direction. The Rx direction is activated upon callbacks from the lower layers. +The Tx direction is activated recursively from the upper layers. Between every +two layers there may be a buffer, in each direction. When a buffer is full +either in Tx or Rx direction, this direction is paused from the network layer +and the location where the congestion is encountered. Upon end of congestion +(cs_recv() from the upper layer, of sendto() at the lower layers), a +tasklet_wakeup() is performed on the blocked layer so that suspended operations +can be resumed. In this case, the Rx side restarts propagating data upwards +from the lowest blocked level, while the Tx side restarts propagating data +downwards from the highest blocked level. Proceeding like this ensures that +information known to the producer may always be used to tailor the buffer sizes +or decide of a strategy to best aggregate data. Additionally, each time a layer +is crossed without transformation, it becomes possible to send without copying. + +The Rx side notifies the application of data readiness using a wakeup or a +callback. The Tx side notifies the application of room availability once data +have been moved resulting in the uppermost buffer having some free space. + +When crossing a mux downwards, it is possible that the sender is not allowed to +access the buffer because it is not yet its turn. It is not a problem, the data +remains in the conn_stream's buffer (or the stream one) and will be restarted +once the mux is ready to consume these data. + + + cs_recv() -------. cs_send() + ^ +--------> |||||| -------------+ ^ + | | -------' | | stream + --|----------|-------------------------------|-------|------------------- + | | V | connection + data .---. | | room + ready! |---| |---| available! + |---| |---| + |---| |---| + | | '---' + ^ +------------+-------+ | + | | ^ | / + / V | V / + / recvfrom() | sendto() | + -------------|----------------|--------------|--------------------------- + | | poll! V kernel + + +The cs_recv() function should act on pointers to buffer pointers, so that the +callee may decide to pass its own buffer directly by simply swapping pointers. +Similarly for cs_send() it is desirable to let the callee steal the buffer by +swapping the pointers. This way it remains possible to implement zero-copy +forwarding. + +Some operation flags will be needed on cs_recv() : + - RECV_ZERO_COPY : refuse to merge new data into the current buffer if it + will result in a data copy (ie the buffer is not empty), unless no more + than XXX bytes have to be copied (eg: copying 2 cache lines may be cheaper + than waiting and playing with pointers) + + - RECV_AT_ONCE : only perform the operation if it will result in the source + buffer to become empty at the end of the operation so that no two buffers + remain allocated at the end. It will most of the time result in either a + small read or a zero-copy operation. + + - RECV_PEEK : retrieve a copy of pending data without removing these data + from the source buffer. Maybe an alternate solution could consist in + finding the pointer to the source buffer and accessing these data directly, + except that it might be less interesting for the long term, thread-wise. + + - RECV_MIN : receive minimum X bytes (or less with a shutdown), or fail. + This should help various protocol parsers which need to receive a complete + frame before proceeding. + + - RECV_ENOUGH : no more data expected after this read if it's of the + requested size, thus no need to re-enable receiving on the lower layers. + + - RECV_ONE_SHOT : perform a single read without re-enabling reading on the + lower layers, like we currently do when receiving an HTTP/1 request. Like + RECV_ENOUGH where any size is enough. Probably that the two could be merged + (eg: by having a MIN argument like RECV_MIN). + + +Some operation flags will be needed on cs_send() : + - SEND_ZERO_COPY : refuse to merge the presented data with existing data and + prefer to wait for current data to leave and try again, unless the consumer + considers the amount of data acceptable for a copy. + + - SEND_AT_ONCE : only perform the operation if it will result in the source + buffer to become empty at the end of the operation so that no two buffers + remain allocated at the end. It will most of the time result in either a + small write or a zero-copy operation. + + +Both operations should return a composite status : + - number of bytes transferred + - status flags (shutr, shutw, reset, empty, full, ...) + + +2018-07-23 - Update after merging rxbuf +--------------------------------------- + +It becomes visible that the mux will not always be welcome to decode incoming +data because it will sometimes imply extra memory copies and/or usage for no +benefit. + +Ideally, when when a stream is instantiated based on incoming data, these +incoming data should be passed and the upper layers called, but it should then +be up these upper layers to peek more data in certain circumstances. Typically +if the pending connection data are larger than what is expected to be passed +above, it means some data may cause head-of-line blocking (HOL) to other +streams, and needs to be pushed up through the layers to let other streams +continue to work. Similarly very large H2 data frames after header frames +should probably not be passed as they may require copies that could be avoided +if passed later. However if the decoded frame fits into the conn_stream's +buffer, there is an opportunity to use a single buffer for the conn_stream +and the channel. The H2 demux could set a blocking flag indicating it's waiting +for the upper stream to take over demuxing. This flag would be purged once the +upper stream would start reading, or when extra data come and change the +conditions. + +Forcing structured headers and raw data to coexist within a single buffer is +quite challenging for many code parts. For example it's perfectly possible to +see a fragmented buffer containing series of headers, then a small data chunk +that was received at the same time, then a few other headers added by request +processing, then another data block received afterwards, then possibly yet +another header added by option http-send-name-header, and yet another data +block. This causes some pain for compression which still needs to know where +compressed and uncompressed data start/stop. It also makes it very difficult +to account the exact bytes to pass through the various layers. + +One solution consists in thinking about buffers using 3 representations : + + - a structured message, which is used for the internal HTTP representation. + This message may only be atomically processed. It has no clear byte count, + it's a message. + + - a raw stream, consisting in sequences of bytes. That's typically what + happens in data sequences or in tunnel. + + - a pipe, which contains data to be forwarded, and that haproxy cannot have + access to. + +The processing efficiency decreases with the higher complexity above, but the +capabilities increase. The structured message can contain anything including +serialized data blocks to be processed or forwarded. The raw stream contains +data blocks to be processed or forwarded. The pipe only contains data blocks +to be forwarded. The the latter ones are only an optimization of the former +ones. + +Thus ideally a channel should have access to all such 3 storage areas at once, +depending on the use case : + (1) a structured message, + (2) a raw stream, + (3) a pipe + +Right now a channel only has (2) and (3) but after the native HTTP rework, it +will only have (1) and (3). Placing a raw stream exclusively in (1) comes with +some performance drawbacks which are not easily recovered, and with some quite +difficult management still involving the reserve to ensure that a data block +doesn't prevent headers from being appended. But during header processing, the +payload may be necessary so we cannot decide to drop this option. + +A long-term approach would consist in ensuring that a single channel may have +access to all 3 representations at once, and to enumerate priority rules to +define how they interact together. That's exactly what is currently being done +with the pipe and the raw buffer right now. Doing so would also save the need +for storing payload in the structured message and void the requirement for the +reserve. But it would cost more memory to process POST data and server +responses. Thus an intermediary step consists in keeping this model in mind but +not implementing everything yet. + +Short term proposal : a channel has access to a buffer and a pipe. A non-empty +buffer is either in structured message format OR raw stream format. Only the +channel knows. However a structured buffer MAY contain raw data in a properly +formatted way (using the envelope defined by the structured message format). + +By default, when a demux writes to a CS rxbuf, it will try to use the lowest +possible level for what is being done (i.e. splice if possible, otherwise raw +stream, otherwise structured message). If the buffer already contains a +structured message, then this format is exclusive. From this point the MUX has +two options : either encode the incoming data to match the structured message +format, or refrain from receiving into the CS's rxbuf and wait until the upper +layer request those data. + +This opens a simplified option which could be suited even for the long term : + - cs_recv() will take one or two flags to indicate if a buffer already + contains a structured message or not ; the upper layer knows it. + + - cs_recv() will take two flags to indicate what the upper layer is willing + to take : + - structured message only + - raw stream only + - any of them + + From this point the mux can decide to either pass anything or refrain from + doing so. + + - the demux stores the knowledge it has from the contents into some CS flags + to indicate whether or not some structured message are still available, and + whether or not some raw data are still available. Thus the caller knows + whether or not extra data are available. + + - when the demux works on its own, it refrains from passing structured data + to a non-empty buffer, unless these data are causing trouble to other + streams (HOL). + + - when a demux has to encapsulate raw data into a structured message, it will + always have to respect a configured reserve so that extra header processing + can be done on the structured message inside the buffer, regardless of the + supposed available room. In addition, the upper layer may indicate using an + extra recv() flag whether it wants the demux to defragment serialized data + (for example by moving trailing headers apart) or if it's not necessary. + This flag will be set by the stream interface if compression is required or + if the http-buffer-request option is set for example. Probably that using + to_forward==0 is a stronger indication that the reserve must be respected. + + - cs_recv() and cs_send() when fed with a message, should not return byte + counts but message counts (i.e. 0 or 1). This implies that a single call to + either of these functions cannot mix raw data and structured messages at + the same time. + +At this point it looks like the conn_stream will have some encapsulation work +to do for the payload if it needs to be encapsulated into a message. This +further magnifies the importance of *not* decoding DATA frames into the CS's +rxbuf until really needed. + +The CS will probably need to hold indication of what is available at the mux +level, not only in the CS. Eg: we know that payload is still available. + +Using these elements, it should be possible to ensure that full header frames +may be received without enforcing any reserve, that too large frames that do +not fit will be detected because they return 0 message and indicate that such +a message is still pending, and that data availability is correctly detected +(later we may expect that the stream-interface allocates a larger or second +buffer to place the payload). + +Regarding the ability for the channel to forward data, it looks like having a +new function "cs_xfer(src_cs, dst_cs, count)" could be very productive in +optimizing the forwarding to make use of splicing when available. It is not yet +totally clear whether it will split into "cs_xfer_in(src_cs, pipe, count)" +followed by "cs_xfer_out(dst_cs, pipe, count)" or anything different, and it +still needs to be studied. The general idea seems to be that the receiver might +have to call the sender directly once they agree on how to transfer data (pipe +or buffer). If the transfer is incomplete, the cs_xfer() return value and/or +flags will indicate the current situation (src empty, dst full, etc) so that +the caller may register for notifications on the appropriate event and wait to +be called again to continue. + +Short term implementation : + 1) add new CS flags to qualify what the buffer contains and what we expect + to read into it; + + 2) set these flags to pretend we have a structured message when receiving + headers (after all, H1 is an atomic header as well) and see what it + implies for the code; for H1 it's unclear whether it makes sense to try + to set it without the H1 mux. + + 3) use these flags to refrain from sending DATA frames after HEADERS frames + in H2. + + 4) flush the flags at the stream interface layer when performing a cs_send(). + + 5) use the flags to enforce receipt of data only when necessary + +We should be able to end up with sequential receipt in H2 modelling what is +needed for other protocols without interfering with the native H1 devs. + + +2018-08-17 - Considerations after killing cs_recv() +--------------------------------------------------- + +With the ongoing reorganisation of the I/O layers, it's visible that cs_recv() +will have to transfer data between the cs' rxbuf and the channel's buffer while +not being aware of the data format. Moreover, in case there's no data there, it +needs to recursively call the mux's rcv_buf() to trigger a decoding, while this +function is sometimes replaced with cs_recv(). All this shows that cs_recv() is +in fact needed while data are pushed upstream from the lower layers, and is not +suitable for the "pull" mode. Thus it was decided to remove this function and +put its code back into h2_rcv_buf(). The H1 mux's rcv_buf() already couldn't be +replaced with cs_recv() since it is the only one knowing about the buffer's +format. + +This opportunity simplified something : if the cs's rxbuf is only read by the +mux's rcv_buf() method, then it doesn't need to be located into the CS and is +well placed into the mux's representation of the stream. This has an important +impact for H2 as it offers more freedom to the mux to allocate/free/reallocate +this buffer, and it ensures the mux always has access to it. + +Furthermore, the conn_stream's txbuf experienced the same fate. Indeed, the H1 +mux has already uncovered the difficulty related to the channel shutting down +on output, with data stuck into the CS's txbuf. Since the CS is tightly coupled +to the stream and the stream can close immediately once its buffers are empty, +it required a way to support orphaned CS with pending data in their txbuf. This +is something that the H2 mux already has to deal with, by carefully leaving the +data in the channel's buffer. But due to the snd_buf() call being top-down, it +is always possible to push the stream's data via the mux's snd_buf() call +without requiring a CS txbuf anymore. Thus the txbuf (when needed) is only +implemented in the mux and attached to the mux's representation of the stream, +and doing so allows to immediately release the channel once the data are safe +in the mux's buffer. + +This is an important change which clarifies the roles and responsibilities of +each layer in the chain : when receiving data from a mux, it's the mux's +responsibility to make sure it can correctly decode the incoming data and to +buffer the possible excess of data it cannot pass to the requester. This means +that decoding an H2 frame, which is not retryable since it has an impact on the +HPACK decompression context, and which cannot be reordered for the same reason, +simply needs to be performed to the H2 stream's rxbuf which will then be passed +to the stream when this one calls h2_rcv_buf(), even if it reads one byte at a +time. Similarly when calling h2_snd_buf(), it's the mux's responsibility to +read as much as it needs to be able to restart later, possibly by buffering +some data into a local buffer. And it's only once all the output data has been +consumed by snd_buf() that the stream is free to disappear. + +This model presents the nice benefit of being infinitely stackable and solving +the last identified showstoppers to move towards a structured message internal +representation, as it will give full power to the rcv_buf() and snd_buf() to +process what they need. + +For now the conn_stream's flags indicating whether a shutdown has been seen in +any direction or if an end of stream was seen will remain in the conn_stream, +though it's likely that some of them will move to the mux's representation of +the stream after structured messages are implemented. |