Hello! On Fri, Jan 27, 2023 at 08:11:34PM +0800, Miao Wang wrote:
> > 2023年1月27日 12:01，Maxim Dounin <mdou...@mdounin.ru> 写道： > > > > On Wed, Jan 25, 2023 at 12:49:10PM +0800, Miao Wang wrote: > > > >>> 2023年1月25日 10:17，Maxim Dounin <mdou...@mdounin.ru> 写道： > >>> > >>> On Mon, Jan 23, 2023 at 07:01:16PM +0800, Miao Wang wrote: > >>> > >>>>> 2023年1月23日 12:05，Maxim Dounin <mdou...@mdounin.ru> 写道： > >>>>> > >>>>> On Wed, Jan 18, 2023 at 11:28:52PM +0800, Miao Wang wrote: > >>>>> > >>>>>> # HG changeset patch > >>>>>> # User Miao Wang <shankerwangm...@gmail.com> > >>>>>> # Date 1674055068 -28800 > >>>>>> # Wed Jan 18 23:17:48 2023 +0800 > >>>>>> # Node ID 73aa64bd29f3dec9e43e97560d6b5a07cdf40063 > >>>>>> # Parent 07b0bee87f32be91a33210bc06973e07c4c1dac9 > >>>>>> HTTP: trigger lingering close when keepalive connection will be closed > >>>>>> > >>>>>> When finalizing a request, if the request is not keepalive but > >>>>>> its connection has served more than one request, then the connection > >>>>>> has been a keepalive connection previously and this connection will > >>>>>> be closed after this response. In this condition, it is likely that > >>>>>> there are pipelined requests following this request, which we should > >>>>>> ignore. As a result, lingering close is necessary in this case. > >>>>>> > >>>>>> Without this patch, nginx (with its default configuration) will send > >>>>>> out TCP RST when there are more pipelined requests. The symptom is > >>>>>> obvious when nginx is serving a debian repository and apt is > >>>>>> downloading massive of packages. See [1]. It becomes more obvious > >>>>>> when `keepalive_requests` is lower or nginx is under a relative > >>>>>> higher load, and it disappears when specifying > >>>>>> `lingering_close always`. > >>>>>> > >>>>>> [1]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=973861#10 > >>>>>> > >>>>>> diff -r 07b0bee87f32 -r 73aa64bd29f3 src/http/ngx_http_request.c > >>>>>> --- a/src/http/ngx_http_request.c Wed Dec 21 14:53:27 2022 +0300 > >>>>>> +++ b/src/http/ngx_http_request.c Wed Jan 18 23:17:48 2023 +0800 > >>>>>> @@ -2749,6 +2749,10 @@ > >>>>>> return; > >>>>>> } > >>>>>> > >>>>>> + if (!r->keepalive && r->connection->requests > 1) { > >>>>>> + r->lingering_close = 1; > >>>>>> + } > >>>>>> + > >>>>>> if (clcf->lingering_close == NGX_HTTP_LINGERING_ALWAYS > >>>>>> || (clcf->lingering_close == NGX_HTTP_LINGERING_ON > >>>>>> && (r->lingering_close > >>>>> > >>>>> Thanks for the patch and the link to the Debian bug report. > >>>>> > >>>>> Lingering close implies noticeable additional resource usage: even > >>>>> if nothing happens on the connection, it will be kept open for > >>>>> lingering_timeout, which is 5 seconds by default. Given that > >>>>> pipelining is not used by most of the clients, forcing lingering > >>>>> close for all clients which are using keepalive does not look like > >>>>> a good solution to me. > >>>>> > >>>>> In general, nginx tries hard to determine if any additional data > >>>>> are expected on the connection, and uses lingering close if there > >>>>> is a good chance there will be some, but avoids lingering close by > >>>>> default if additional data are unlikely. If this logic does not > >>>>> work for some reason, lingering close can be explicitly requested > >>>>> with "lingering_close always;". > >>>> > >>>> That's true since the symptom I described can be worked around with > >>>> that option. > >>>> > >>>>> > >>>>> In particular, note the "r->header_in->pos < r->header_in->last" > >>>>> and "r->connection->read->ready" checks - these are expected to > >>>>> catch connections with additional pipelined requests (see revision > >>>>> 3981:77604e9a1ed8). And from the log provided in the report it > >>>>> looks like it works most of the time - there are more than 6k HTTP > >>>>> requests, and 60+ connections. But sometimes it fails - there are > >>>>> two RST errors logged (and one "Undetermined Error", which looks > >>>>> like a bug in apt, but might be related). > >>>>> > >>>>> It looks like when apt is downloading many resources, it does not > >>>>> send all the requests at once (or in batches), but instead tries > >>>>> to maintain a constant "depth", a number of pipelined requests in > >>>>> flight. This essentially means that after reading of a response > >>>>> it sends an additional request. > >>>> > >>>> That's right. From a traffic dump, I can see apt first sends one > >>>> request, and after receiving the response, it will send out 10 > >>>> more requests, and maintain a depth of 10, since by default > >>>> Acquire::http::Pipeline-Depth is 10. > >>>> > >>>>> > >>>>> I see at least two possible cases which can result in nginx not > >>>>> using lingering close with such a load: > >>>>> > >>>>> 1. If a response where keepalive_requests is reached happens to > >>>>> be the last request in the r->header_in buffer (so the > >>>>> "r->header_in->pos < r->header_in->last" won't be effective), and > >>>>> there is a chance that nginx wasn't yet got an event from kernel > >>>>> about additional data (and therefore "r->connection->read->ready" > >>>>> will not be set). As such, nginx won't use lingering close, and > >>>>> might close connection with unread data in the socket buffer, > >>>>> resulting in RST. > >>>>> > >>>>> 2. Similarly, if nginx happens to be faster than apt, and socket > >>>>> buffers are large enough, it might sent all the responses, > >>>>> including the last one with "Connection: close", and close the > >>>>> connection (since there are no pending pipelined requests at the > >>>>> moment) even before an additional request is sent by apt. When > >>>>> later apt will send an additional request after reading some of > >>>>> the responses, it will send the request to already closed > >>>>> connection, again resulting in RST. > >>>> > >>>> Actually, comparing the debug log and the pcap, nginx calls > >>>> close() after writing the last response. However, at that time, > >>>> that response is not fully transmitted to the client and there > >>>> seems to be more requests not processed in the kernel buffer. > >>>> Thus close() triggers an immediate RST. > >>> > >>> Thanks for the details. This looks more like the first case, and > >>> probably can be addressed by improving likelihood of detecting the > >>> read event. > >>> > >>> Could you please test if the patch below fixes the particular > >>> issue you are seeing? It is somewhat unrelated, but it might be > >>> a good enough solution (and is more or less equivalent to > >>> checking r->pipeline). > >>> > >>>>> It would be interesting to see more details, such as tcpdump > >>>>> and/or nginx debug logs, to find out what actually goes on here. > >>>> > >>>> The tcpdump and debug logs are too large to send in this mail list. > >>>> I wonder if I can directly email it to you. > >>> > >>> Feel free to, my email should accept up to 100M messages. > >>> Alternatively, a good solution might be to make the files > >>> available for download and post a link here. > >>> > >>>>> Overall, given how apt uses pipelining, I tend to think that at > >>>>> least (2) is unavoidable and can happen with certain sizes of the > >>>>> responses. > >>>>> > >>>>> A good enough solution might be check for r->pipeline, which is > >>>>> set by nginx as long as it reads a pipelined request. It might > >>>>> not be enough though, since r->pipeline is only set for requests > >>>>> seen by nginx as pipelined, which might not be true for the last > >>>>> request. > >>>>> > >>>>> A more complete solution might be to introduce something like > >>>>> c->pipeline flag and use lingering close if any pipelined requests > >>>>> were seen on the connection. > >>> > >>> The following patch reworks handling of pipelined requests by > >>> postponing them to the next event loop iteration. It is expected > >>> make it more likely for nginx to know there are any additional > >>> unread data in the socket buffer (and right now is mostly > >>> equivalent to checking r->pipeline, since c->read->ready is always > >>> set for pipelined requests): > >>> > >>> # HG changeset patch > >>> # User Maxim Dounin <mdou...@mdounin.ru> > >>> # Date 1674610218 -10800 > >>> # Wed Jan 25 04:30:18 2023 +0300 > >>> # Node ID 8cfd22c325a3db370b9e45aa6f897ff7bc8222f3 > >>> # Parent c7e103acb409f0352cb73997c053b3bdbb8dd5db > >>> Reworked pipelined requests to use posted next events. > >>> > >>> This is expected to improve handling of pipelined requests in a number > >>> of ways, including: > >>> > >>> 1) It will make a room for additional requests from other clients, > >>> reducing worker monopolization by a single client. > >>> > >>> 2) The c->read->ready flag will be set, so nginx will either read the > >>> additional data, or will use lingering close. This is expected to help > >>> with clients using pipelining with some constant depth, such as apt[1][2]. > >>> > >>> The ngx_event_move_posted_next() was modified to make it possible to > >>> post read events on connections with kqueue. Previously, it used to > >>> set ev->available to -1, potentially overwriting a valid positive value > >>> provided by kqueue, so ngx_unix_recv() and ngx_readv_chain() will stop > >>> reading from the socket before reading all the data available. > >>> > >>> Note that currently ngx_event_move_posted_next() will always set > >>> the ev->ready flag. While this is expected behaviour for the > >>> ev->available > >>> use case (where ev->ready is explicitly cleared), this is not needed for > >>> pipelining. For pipelining, this will result in extra unneeded read() > >>> syscall after processing of all pipelined requests, and there might be > >>> a room for improvement here. > >>> > >>> [1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=973861#10 > >>> [2] > >>> https://mailman.nginx.org/pipermail/nginx-devel/2023-January/ZA2SP5SJU55LHEBCJMFDB2AZVELRLTHI.html > >>> > >>> diff --git a/src/event/ngx_event_posted.c b/src/event/ngx_event_posted.c > >>> --- a/src/event/ngx_event_posted.c > >>> +++ b/src/event/ngx_event_posted.c > >>> @@ -51,8 +51,10 @@ ngx_event_move_posted_next(ngx_cycle_t * > >>> ngx_log_debug1(NGX_LOG_DEBUG_EVENT, cycle->log, 0, > >>> "posted next event %p", ev); > >>> > >>> - ev->ready = 1; > >>> - ev->available = -1; > >>> + if (!ev->ready) { > >>> + ev->ready = 1; > >>> + ev->available = -1; > >>> + } > >>> } > >>> > >>> ngx_queue_add(&ngx_posted_events, &ngx_posted_next_events); > >>> diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c > >>> --- a/src/http/ngx_http_request.c > >>> +++ b/src/http/ngx_http_request.c > >>> @@ -3129,7 +3129,7 @@ ngx_http_set_keepalive(ngx_http_request_ > >>> } > >>> > >>> rev->handler = ngx_http_process_request_line; > >>> - ngx_post_event(rev, &ngx_posted_events); > >>> + ngx_post_event(rev, &ngx_posted_next_events); > >>> return; > >>> } > >>> > >> > >> I can confirm that the symptom disappears after applying this patch > > > > Thanks for testing, and thanks for the logs and traffic dumps > > provided. > > > > From the logs you've provided it looks like the patch should help > > with 1 out of 5 errors: the last request in the connection with > > this error was pipelined, and therefore with the patch it is > > expected to use lingering close. > > > > In 4 other cases the last requests are handled without pipelining: > > once the request arrives, ngx_http_keepalive_handler() is called, > > and nginx reads the request and responds to it, closing the > > connection. The next request arrives at some time later, and the > > OS responds with RST. > > > > E.g., the request to bzip2_1.0.8-4_amd64.deb (which corresponds to > > the second error as seen by apt, while downloading > > libpython3.9-stdlib_3.9.2-1_amd64.deb; connection *26, with client > > port 40110) arrives at 14:08:36.495858, nginx responds to it, as > > seen in nginx logs, and then closes the connection. The next > > request arrives at 14:08:36.496977 and immediately responded with > > RST. > > > > The patch does not change handling of these 4 cases, except may > > be some mostly unrelated event handling changes, so the timing > > might be slightly different. As such, I would expect the errors > > to be still reproducible with the patch, but probably somewhat > > less often. > > > > Could you please re-check to see if the symptoms still happen with > > the patch, at least occasionally? > > You are totally right about that. I tested the previous patch > for more times and can see the RST occasionally with less frequency. Thanks for confirming this. > > Overall, after looking into logs and tcpdump you've provided I > > tend to think that the only working fix would be to introduce > > c->pipeline flag, and force lingering close if there were any > > pipelined requests on the connection. > > > > Below is the patch which implements this approach. Review and > > testing appreciated. It can be used either separately or with the > > previously provided patch to use posted next events. > > I've tested the new patch for several times and can confirm the > problem is solved. Also, I added a new variable exposing the > r->connection->pipeline and can confirm that the flag works > as intended. The flag won't be set for normal keep-alive requests, > and will only be set for pipelined requests. > > Thanks again for looking into this issue. Thanks for testing. -- Maxim Dounin http://mdounin.ru/ _______________________________________________ nginx-devel mailing list nginx-devel@nginx.org https://mailman.nginx.org/mailman/listinfo/nginx-devel
