[DNM] tracing: use LTTng for logging #22458

mogeb · 2018-06-07T15:58:13Z

See src/tracing/tracetool/README for details. Use -DWITH_LTTNG_LOGGING=ON with cmake to use LTTng for logging.
Using fio's objectstore backend, with the following job:

runtime=120
rw=randwrite
bs=4m
numjobs=16
iodepth=16

I get the following results:

with dout and debug bluestore = 10/0:

Average throughput = 119 MiB/s
Average IOPS = 13.68
Disk usage: 48M

with LTTng:

Average throughput = 125 MiB/s
Average IOPS = 14.09
Disk usage: 23M

fio is writing to Bluestore on an SSD and the log is being written to an HDD for both dout and LTTng. The throughput difference varies. I've seen LTTng up to 10% faster with more iodepth and numjobs (more contention on dout's internal structures), and I've seen it pretty much the same as dout (with lower iodepth and lower numjobs). However, the size on disk of the log is consistently 50%-60% smaller with LTTng (as expected).

liewegas · 2018-06-07T17:55:01Z

src/ceph_osd.cc

+  chan.attr.read_timer_interval = 200;
+  chan.attr.output = LTTNG_EVENT_MMAP;
+
+  dout(1) << "mogeb creating session " << tracing_session << " in " << (string("/var/data/ceph/build/out/") + tracing_session) << dendl;


maybe replace "mogeb" with something else? :)

mogeb · 2018-06-07T18:02:09Z

src/bootstrap-tracing.sh

@@ -0,0 +1,28 @@
+#!/bin/bash


This file can be ignored, I've been using it locally so I don't have to do_cmake every time to switch between backends.

chardan · 2018-06-07T19:22:38Z

src/common/hobject.h

@@ -445,6 +445,18 @@ struct ghobject_t {
    (*this) = temp;
  }

+  operator std::string() {
+    std::stringstream out;
+    out << this;


It looks like this will write the address of "this" to the stream, rather than the value, *this. Is that what you want? Is there some other magic happening here that writes the value instead? Do I just need coffee? :-)
(There are a number of these, so I'm guessing that /is/ what you want, but I saw at least one exception so I figure it's worth asking!)

OT, but these operator whatever() methods make me very nervous. IIUC, this will silently cast/convert the object to a string if it's in a context that wants a string (e.g., passing to void func(string foo)), right? I've been bitten by silent/accidental conversions in the past. Do we need these, or would an explicit std::string str() method suffice?

That is correct, it's for the implicit cast to string for unknown types. A simple str() method would also work, and we'd append the .str() call in tracetool.

chardan · 2018-06-07T19:29:48Z

src/osd/osd_types.h

@@ -3792,6 +3809,7 @@ class pg_missing_const_i {
  virtual bool is_missing(const hobject_t& oid, pg_missing_item *out = nullptr) const = 0;
  virtual bool is_missing(const hobject_t& oid, eversion_t v) const = 0;
  virtual ~pg_missing_const_i() {}
+  virtual operator std::string() { return "needs implementation"; }


Given that this class is already pure virtual, why not just make this pure virtual as well?

You're obviously right. I was either not thinking, or this was a note to myself to look into implementing the operator in the base class. Either way, thanks!

tchaikov · 2018-06-30T13:57:14Z

src/common/hobject.h

+    return out.str();
+  }
+
+  operator std::string() const {


why do we need two operator std::string()? i think the const one would suffice.

tchaikov · 2018-07-05T15:08:53Z

hey @mogeb may i learn your plan on this PR? please let me know if you need more hands. probably i can help on it.

mogeb · 2018-07-05T15:18:11Z

@tchaikov sorry for the delay, I got side-tracked on another project for a couple of weeks now, but I'm planning on coming back to this PR next week.

I already have a second iteration of this PR ready and working, I just need to clean it a bit and probably ask you for a code review and to try it for usability. The second iteration gets rid of the tracing/tracetool/subsys files, as I explained here. If you have any thought that approach, please feel free to share it.

mogeb · 2018-07-11T21:37:29Z

@tchaikov I've updated the PR. This is still experimental so please excuse some less than optimal code. You should be able to try it, and I think it's better to look at the usability before doing a full code review. For now, only Bluestore.cc can be instrumented, I've updated the README accordingly.

tchaikov · 2018-07-18T04:15:31Z

src/include/config-h.in.cmake

@@ -178,6 +178,9 @@
 /* Define if you want to use LTTng */
 #cmakedefine WITH_LTTNG

+/* Degine if you want to use LTTng for logging instead of dout */


s/Degine/Define/

tchaikov · 2018-07-18T04:15:45Z

src/include/interval_set.h

@@ -723,6 +723,13 @@ class interval_set {
    other = std::move(m);
  }

+public:
+  operator std::string() {


mark it const if you please.

tchaikov · 2018-07-18T04:26:42Z

src/common/hobject.cc

@@ -355,7 +355,11 @@ int cmp(const hobject_t& l, const hobject_t& r)
  return 0;
 }

-
+ghobject_t::operator std::string() const {


nit, we could use CRTP for this idiom, like

template <typename T> struct Printable { operator std::string() const { std::stringstream out; out << *static_cast<const T*>(this); return out.str(); } }; struct Bar : Printable<Bar> {}; ostream& operator<<(ostream& os, const Bar&) { return os << "bar!"; }

so we don't need to repeat this method for every class that needs to be printed in the new logging system.

Yes that would make sense. OTOH this comment suggests to use a str() method instead of relying on the implicit operator std::string() to avoid surprises. What are your thoughts?

i am also for the str() method, or at least make the operator std::string explicit, like

explicit operator std::string() const { /// .... }

. @mogeb have you considered reusing a single trace point everywhere and send the fmt string to it, i.e. use it as an alternative of syslog(3) without the ... parameters. if we are fine with this, we can avoid all these troubles. but the downside of this apporach.

@tchaikov sure, that's the very first solution I explored (pre-Cephalocon). I initially changed the implementation of dout() to just record a tracepoint. With that approach, none of the actual has to be changed. Another alternative could be to use LTTng's tracef() (similar to printf), but it also needs/uses strings.

My objective here though is to avoid passing strings around for performance reasons.

Right. I think if the legacy dout lines degenerate into a slow string op, we can focus our efforts on replacing dout calls in the fast path and leave the less frequently-run and slow-path code alone until later (e.g., peering).

thanks guys. after re-reading tracing/tracetool/{README.md,parsetool.py}, now i understand the objective and rationale behind this design.

@mogeb i agree with @liewegas on this. and i was thinking about how to ease the pain to add a logging message in ceph in future. i even explored the possibility to use libclang to parse the source for creating the .tp file. with it, we can coin the event name using the filename, and lineno, and deduce the name of argument type from the argument in the trace_*(...) expression, so we can extract information interesting to us and create the .tp , .h and _impl.h files for us. but this dependency is kind of heavy weight for us at this moment. probably we can revisit it in future.

but, we can always have a compromise here by differentiating following two use cases:

the logging in critical path: to use trace_${event_name}(loglevel, subsys, [arg_type, arg_name]...), so we don't need to render the values of primitive types in string before passing them to LTTng.

the logging in non-critical path: to use trace(loglevel, subsys, fmt_str, [arg]...). we can use fmt::format() offered by libfmt in the pasrsetool.py. as libfmt is already included by ceph as a submodule. and make trace() a template function. which calls tracepoint(subsys, ${default_event_name}_at_${log_evel}) , assuming all combinations of ${default_event_name}_at_${log_level} TRACEPOINT_EVENTs of different ${subsys}s and ${log_level}s combinations have been defined by parsetool.py. they only have a single TP_FIELD.

in this way, we can have

a unified logging backend, so developers or users don't need to check different sources for tracing an issue, and

a flexible front-end, so developers don't need to worry about the name of a trace event, and the argument type of event payload for all logging messages. they only need to do so when it's really needed.

what do you think?

I think the clang approach would be great. Sounds good, let's rethink it later as you said.

For having a critical and non-critical paths: just to make sure I understand, the non-critical path uses a formatted string using the arguments' operator <<() functions, correct? If that's the case, we could change the implementation of dout to record a tracepoint instead of using trace(loglevel, subsys, fmt_str, [arg]...). This becomes even easier for the developers as nothing changes: dout() just takes the string passed to it and ends up calling tracepoint() at the end. I'd need to figure out the details, but I was able to achieve that at the very beginning. Any thought?

the non-critical path uses a formatted string using the arguments' operator <<() functions, correct?

yes. at this moment, we can use the argument's operator<<(), so minimum change is needed. in short and long term, i'd suggest use libfmt's fmt::format() instead, it has better performance than that of std::iostream. probably we can expose two more interfaces

just for the backward compatibility: `dout(1) << "ceph rock" << 5 << '!' << dendl;

as the faster one: trace(bluestore, 1, "{0} rock{1}{2}", "ceph", 5, '!');

as the fatest one: trace_ceph_rocks(1, bluestore, string, who, "ceph", int, s, 5, uint_8, wow, '!', "%s rock %d%d");

so, i'd suggest start using the trace() or trace_${event_name} interface in the new code. and replace the implementaion of dout(1) for the existing code.

tchaikov · 2018-07-31T16:01:48Z

src/tracing/tracetool/parsetool.py

+                'static inline void __%(api)s(%(args)s)',
+                '{',
+                api=e.api(e.CEPH_TRACE),
+                args=", ".join([' '.join(arg[:2]) if is_valid_type(arg[0]) else ' '.join(('string', arg[1])) for arg in e.args]))


might want to differentiate the args which are already std::string or string in Event.build(). probably we could have three Arg types to represent them?

so we don't create a temporary copy of them just for passing them to ctf_string().

tchaikov · 2018-07-31T16:02:57Z

@mogeb i will try your patch out. probably the best way to improve it is to use it.

tchaikov · 2018-08-01T09:20:07Z

src/osd/CMakeLists.txt

+if(WITH_LTTNG_LOGGING)
+  add_dependencies(osd bluestore-parsetool)
+  target_link_libraries(osd logging_tp)
+  target_link_libraries(osd lttng-ust-fork)


might want to add dependencies from os to *_impl.h, as BlueStore.cc now includes them. could use set_source_files_properties() for this purpose.

Noted, thanks.

mogeb · 2018-08-01T11:47:09Z

@tchaikov any thought on the usability part? The LTTng usage, the resulting trace, reading/parsing it, etc.

tchaikov · 2018-08-02T10:22:42Z

src/ceph_osd.cc

@@ -19,6 +19,7 @@

 #include <iostream>
 #include <string>
+#include <lttng/lttng.h>


might want to add liblttng-ctl-dev as a build dependency.

tchaikov · 2018-08-06T12:39:48Z

@mogeb i am stilling playing around with it. will reply you tomorrow.

tchaikov · 2018-08-07T11:41:07Z

@mogeb following is what we have in the output of lttng view after applying this changeset:

...
[18:07:11.985680268] (+0.000163564) gen8 bluestore:do_write_small: { cpu_id = 4 }, { offset = 0x0, length = 0x1DC }
[18:07:11.985854796] (+0.000174528) gen8 bluestore:write: { cpu_id = 4 }, { cid = "meta", oid = "#-1:c683972f:::inc_osdmap.12:0#", offset = 0x0, length = 0x1D
C, r = 0 }
...

i think this the best compact representation of a (log) event we can have in the fast path, but from a typical user's point of view, it's still not very friendly. we can ship the .tp files in a separated package accompanied by a set of command line tools for analyzing the tracings. one of these tools will be able to act as a filter to utilize the babeltrace parser to compose the tracing events and the .tp file to produce a more readable output, in other words, to printf the fields along with the format string.

also, to integrate with existing logging infrastructure:

to see if we can use lttng rotate along with logrotate
update the systemd script to start/stop lttng sessions and tracing

but we can leave these tooling for following-up PRs. i guess, the first thing we need to have is dout backend and the "faster" mode, and to fix the CMakeLists.txt, to make sure it compiles if WITH_LTTNG_LOGGING is enabled. we can keep WITH_LTTNG_LOGGING disabled by default before we get these tooling and script sorted out.

once this change gets merged. we will

improve the tooling,
update the systemd scripts.
fix the packaging dependencies

and then switch OSD over to LTTng based logging.

what do you think?

mogeb · 2018-08-07T12:15:51Z

@tchaikov I really like the approach, especially having a tool that knows how to rewrite the trace's events as more human-readable ones. Note however that the .tp files don't have the printf information in them (how to format and what to print), so we'll need to add a file for it. The session rotation option in the 2.10 version of LTTng certainly makes things easier. We can set the rotation to be automatic and either time-based or size-based. Did you measure any performance difference other than what I had reported? I think the reduced size on disk is a given, but let's confirm that the performance is also better to warrant this added complexity. Once I clean this PR I will run benchmarks on a real cluster too.

A couple of small things to clear:

For the slow path, do we want to keep dout as it is (plain text logging), or do we want to make dout use LTTng, but with the complete formatted string in the payload of the event?
Which log entries do we rewrite as fast-path? Maybe the ones lower than a certain loglevel?

tchaikov · 2018-08-07T12:54:14Z

Did you measure any performance difference other than what I had reported?

@mogeb not yet. i will try to reproduce your test in my test bed.

For the slow path, do we want to keep dout as it is (plain text logging), or do we want to make dout use LTTng, but with the complete formatted string in the payload of the event?

i think it'd be hard for us to read the logging messages if we have a hybrid logging: plain text logging + LTTng tracing. to ease the transition. i'd recommend a runtime option for switching between the plain-text backend and the LTTng backend.

Which log entries do we rewrite as fast-path? Maybe the ones lower than a certain loglevel?

i don't think there is a simple criteria for telling if a log entry is in the fast path by looking at its loglevel. for instance, we use dout(1) for a critical warning or very important message, for example,

dout(1) << "not feeling well.. i am about to re-boot" << dendl;

and use dout(30) for a trivial message,

dout(30) << "tick, tick. i am healthy!" << dendl;

i think, to ease the pain of migration, before we have the tool to "rewrite the trace's events as more human-readable ones.", we'd better leave the logging messages as they are. and leave the tool and the changes (for instance, ff26e95, 2b00fe2, 8e227fb) to LTTngize logging message for another PR.

tchaikov · 2018-08-14T12:57:55Z

@mogeb in my case, lttng based log outperforms the existing dout(). tested with test/fio/ceph-bluestore.fio. a separated SSD for osd_data. and a separated HDD for log/ust.

lttng enable-event  -u -a

  WRITE: bw=65.0MiB/s (68.2MB/s), 65.0MiB/s-65.0MiB/s (68.2MB/s-68.2MB/s), io=1302MiB (1365MB), run=20018-20018msec

while with existing dout backend

  WRITE: bw=36.2MiB/s (37.0MB/s), 36.2MiB/s-36.2MiB/s (37.0MB/s-37.0MB/s), io=725MiB (760MB), run=20017-20017msec

mogeb · 2018-08-14T13:03:54Z

@tchaikov so the performance is twice as good with LTTng compared to current dout? This is great. What's the loglevel you set for dout?

Something came up on my side for so I got sidetracked for a few days, but I'll update this PR afterwards.

Edit: Sorry, I missed a part of your comment at first ;)

tchaikov · 2018-08-14T13:08:18Z

so the performance is twice as good with LTTng compared to current dout?

looks like.

Is that using an actual OSD or fio with Bluestore backend?

fio w/ bluestore backend.

What's the loglevel you set for dout?

debug-bluestore = 10/0

In my tests, the logs were going to a different disk (which is more realistic) than the data with fio. I assume dout does more writes to disk than LTTng, so there's more contention. If the logs are going to the same disk than the data, the performance difference will be more flagrant. Nonetheless, this is good news, thanks.

will double check my settings. and rerun the test .

Something came up on my side for so I got sidetracked for a few days, but I'll update this PR afterwards.

sure. thanks!

cbodley · 2018-08-14T14:18:22Z

src/test/fio/fio_ceph_objectstore.cc

+#ifndef typeof
+#define typeof __typeof__
+#endif
+


not sure why this is necessary. src/test/fio/CMakeLists.txt has a workaround for this:

# fio headers use typeof(), which requires c++11 extensions set_target_properties(fio_ceph_objectstore PROPERTIES CXX_EXTENSIONS ON COMPILE_FLAGS "${FIO_CFLAGS}")

This probably won't be it in the final version of this PR. However, I was planning on submitting it alone as fio_ceph_objectstore.cc wouldn't build in my environment. I haven't tried it since though.

i also ran into this issue while compiling libfio_ceph_objectstore.so with GCC 8.2, hence axboe/fio#628 , which renders both workaround not necessary anymore.

tchaikov · 2018-08-21T15:08:20Z

what i don't understand is why my SSD is 2x slower than it was last week... it was
bw=65.0MiB/s with LTTng logging, while it's now bw=34.7MiB/s with the same settings.

mogeb · 2018-08-21T16:22:42Z

@tchaikov exactly.. even if a trace wasn't generated last time (tracing disabled), it still wouldn't cause that much of a difference. In any case, I will do proper benchmarking once I'm back on this.

tchaikov · 2018-08-29T13:35:07Z

@mogeb ping?

mogeb · 2018-09-11T14:45:12Z

@tchaikov there are now three ways to write an event:

Fast path: trace_<event_name>(loglevel, subsys, type1, arg1, value1 [, type2, arg2, value2, ...], <fmt>);
Slow path: trace(loglevel, subsys, <libfmt-formatted string>, arg1, ...)
Slow path: trace_error(subsys, <libfmt-formatted string>, arg1, ...)

3 is exactly like 2, but the loglevel is not required. For 1, there is no string manipulation, and raw basic types have to be provided. The result in the trace is a compact event. For 2 and 3, arguments are formatted to a string and passed to the event. Here's an example.

The only issue right now is that for 2 and 3, you have to use fmt::ptr(arg) if arg is a pointer, as explained here. It doesn't look like there's a straight way to print pointers (at least in C++).

mogeb · 2018-09-11T14:51:38Z

src/os/bluestore/BlueStore.cc

-  dout(10) << __func__ << " " << cid << " " << oid
-	   << " 0x" << std::hex << offset << "~" << length << std::dec
-	   << " = " << r << dendl;
+  trace(10, "bluestore", "read {}, {}, {}, {} and {}",


We could also replace susbsys with __func__.

mogeb · 2018-10-05T09:59:11Z

Rebased. I'm trying to run this on a real cluster now.

tchaikov · 2018-10-16T14:18:41Z

sorry for the latency, i will review this PR early tomorrow.

mogeb · 2018-10-16T14:25:20Z

@tchaikov no problem, take your time. On my side, I'm trying to build it with a proper build system to deploy it on a real cluster and see what impact it has. I'll update you with the results soon.

mogeb · 2018-11-08T14:58:48Z

Update: my hardware is currently broken so I'm waiting for it to be fixed to report performance numbers on a real cluster. I'll report soon and sorry for the delay.

Signed-off-by: Mohamad Gebai <mgebai@suse.com>

mogeb · 2019-06-25T19:15:32Z

New results here

jdurgin · 2019-08-09T17:08:24Z

This was discussed at the perf weekly 2019-06-20, where @mogeb shared those results. Given the difficulty of use of lttng for both users and developers, we decided that it would make more sense to concentrate on optimizing the log statements in the hot path (e.g. reducing the memcpys and stringification of many structures there) rather than converting the logging machinery to lttng.

I'd update the trello card earlier: https://trello.com/c/WAu7nsH4/362-osd-make-logging-on-the-i-o-path-lightweight but this PR being open caused some confusion, so closing it now.

cbodley · 2019-08-09T17:27:58Z

@mogeb thanks for all of your efforts in exploring this!

mogeb · 2019-08-12T16:03:20Z

@cbodley and @jdurgin thanks for your help as well.

mogeb changed the title ~~[DNM] Use LTTng for logging~~ [DNM] tracing: use LTTng for logging Jun 7, 2018

tchaikov self-requested a review June 7, 2018 15:59

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tchaikov added this to In progress in crimson (moved to https://trello.com/b/lbHkWKxh/crimson) Jun 9, 2018

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tchaikov mentioned this pull request Sep 7, 2018

common,auth,crimson: add logging to crimson #23957

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tracing: add tracetool.py and convert tracepoints

0562eed

Signed-off-by: Mohamad Gebai <mgebai@suse.com>

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[DNM] tracing: use LTTng for logging #22458

[DNM] tracing: use LTTng for logging #22458

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