Radai, 11. The KIP talks about a new server configuration parameter *memory.pool.class.name <http://memory.pool.class.name> *which is not in the implementation. Is it still the case that the pool will be configurable?
12. Personally I would prefer not to have a garbage collected pool that hides bugs as well. Apart from the added code complexity and extra thread to handle collections, I am also concerned about the non-deterministic nature of GC timings. The KIP introduces delays in processing requests based on the configuration parameter *queued.max.bytes. *This in unrelated to the JVM heap size and hence pool can be full when there is no pressure on the JVM to garbage collect. The KIP does not prevent other timeouts in the broker (eg. consumer session timeout) because it is relying on the pool to be managed in a deterministic, timely manner. Since a garbage collected pool cannot provide that guarantee, wouldn't it be better to run tests with a GC-pool that perhaps fails with a fatal error if it encounters a buffer that was not released? 13. The implementation currently mutes all channels that don't have a receive buffer allocated. Would it make sense to mute only the channels that need a buffer (i.e. allow channels to read the 4-byte size that is not read using the pool) so that normal client connection close() is handled even when the pool is full? Since the extra 4-bytes may already be allocated for some connections, the total request memory has to take into account *4*numConnections* bytes anyway. On Thu, Nov 10, 2016 at 11:51 PM, Jun Rao <j...@confluent.io> wrote: > Hi, Radai, > > 1. Yes, I am concerned about the trickiness of having to deal with wreak > refs. I think it's simpler to just have the simple version instrumented > with enough debug/trace logging and do enough stress testing. Since we > still have queued.max.requests, one can always fall back to that if a > memory leak issue is identified. We could also label the feature as beta if > we don't think this is production ready. > > 2.2 I am just wondering after we fix that issue whether the claim that the > request memory is bounded by queued.max.bytes + socket.request.max.bytes > is still true. > > 5. Ok, leaving the default as -1 is fine then. > > Thanks, > > Jun > > On Wed, Nov 9, 2016 at 6:01 PM, radai <radai.rosenbl...@gmail.com> wrote: > > > Hi Jun, > > > > Thank you for taking the time to review this. > > > > 1. short version - yes, the concern is bugs, but the cost is tiny and > worth > > it, and its a common pattern. long version: > > 1.1 detecting these types of bugs (leaks) cannot be easily done with > > simple testing, but requires stress/stability tests that run for a long > > time (long enough to hit OOM, depending on leak size and available > memory). > > this is why some sort of leak detector is "standard practice" .for > example > > look at netty (http://netty.io/wiki/reference-counted-objects. > > html#leak-detection-levels) > > <http://netty.io/wiki/reference-counted-objects. > html#leak-detection-levels > > >- > > they have way more complicated built-in leak detection enabled by > default. > > as a concrete example - during development i did not properly dispose of > > in-progress KafkaChannel.receive when a connection was abruptly closed > and > > I only found it because of the log msg printed by the pool. > > 1.2 I have a benchmark suite showing the performance cost of the gc > pool > > is absolutely negligible - > > https://github.com/radai-rosenblatt/kafka-benchmarks/ > > tree/master/memorypool-benchmarks > > 1.3 as for the complexity of the impl - its just ~150 lines and pretty > > straight forward. i think the main issue is that not many people are > > familiar with weak refs and ref queues. > > > > how about making the pool impl class a config param (generally good > > going forward), make the default be the simple pool, and keep the GC one > as > > a dev/debug/triage aid? > > > > 2. the KIP itself doesnt specifically treat SSL at all - its an > > implementation detail. as for my current patch, it has some minimal > > treatment of SSL - just enough to not mute SSL sockets mid-handshake - > but > > the code in SslTransportLayer still allocates buffers itself. it is my > > understanding that netReadBuffer/appReadBuffer shouldn't grow beyond 2 x > > sslEngine.getSession().getPacketBufferSize(), which i assume to be > small. > > they are also long lived (they live for the duration of the connection) > > which makes a poor fit for pooling. the bigger fish to fry i think is > > decompression - you could read a 1MB blob into a pool-provided buffer and > > then decompress it into 10MB of heap allocated on the spot :-) also, the > > ssl code is extremely tricky. > > 2.2 just to make sure, youre talking about Selector.java: while > > ((networkReceive = channel.read()) != null) addToStagedReceives(channel, > > networkReceive); ? if so youre right, and i'll fix that (probably by > > something similar to immediatelyConnectedKeys, not sure yet) > > > > 3. isOutOfMemory is self explanatory (and i'll add javadocs and update > the > > wiki). isLowOnMem is basically the point where I start randomizing the > > selection key handling order to avoid potential starvation. its rather > > arbitrary and now that i think of it should probably not exist and be > > entirely contained in Selector (where the shuffling takes place). will > fix. > > > > 4. will do. > > > > 5. I prefer -1 or 0 as an explicit "OFF" (or basically anything <=0). > > Long.MAX_VALUE would still create a pool, that would still waste time > > tracking resources. I dont really mind though if you have a preferred > magic > > value for off. > > > > > > > > > > > > On Wed, Nov 9, 2016 at 9:28 AM, Jun Rao <j...@confluent.io> wrote: > > > > > Hi, Radai, > > > > > > Thanks for the KIP. Some comments below. > > > > > > 1. The KIP says "to facilitate faster implementation (as a safety net) > > the > > > pool will be implemented in such a way that memory that was not > > release()ed > > > (but still garbage collected) would be detected and "reclaimed". this > is > > to > > > prevent "leaks" in case of code paths that fail to release() > properly.". > > > What are the cases that could cause memory leaks? If we are concerned > > about > > > bugs, it seems that it's better to just do more testing to make sure > the > > > usage of the simple implementation (SimpleMemoryPool) is solid instead > of > > > adding more complicated logic (GarbageCollectedMemoryPool) to hide the > > > potential bugs. > > > > > > 2. I am wondering how much this KIP covers the SSL channel > > implementation. > > > 2.1 SslTransportLayer maintains netReadBuffer, netWriteBuffer, > > > appReadBuffer per socket. Should those memory be accounted for in > memory > > > pool? > > > 2.2 One tricky thing with SSL is that during a KafkaChannel.read(), > it's > > > possible for multiple NetworkReceives to be returned since multiple > > > requests' data could be encrypted together by SSL. To deal with this, > we > > > stash those NetworkReceives in Selector.stagedReceives and give it back > > to > > > the poll() call one NetworkReceive at a time. What this means is that, > if > > > we stop reading from KafkaChannel in the middle because memory pool is > > > full, this channel's key may never get selected for reads (even after > the > > > read interest is turned on), but there are still pending data for the > > > channel, which will never get processed. > > > > > > 3. The code has the following two methods in MemoryPool, which are not > > > described in the KIP. Could you explain how they are used in the wiki? > > > isLowOnMemory() > > > isOutOfMemory() > > > > > > 4. Could you also describe in the KIP at the high level, how the read > > > interest bit for the socket is turned on/off with respect to > MemoryPool? > > > > > > 5. Should queued.max.bytes defaults to -1 or Long.MAX_VALUE? > > > > > > Thanks, > > > > > > Jun > > > > > > On Mon, Nov 7, 2016 at 1:08 PM, radai <radai.rosenbl...@gmail.com> > > wrote: > > > > > > > Hi, > > > > > > > > I would like to initiate a vote on KIP-72: > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-72%3A+ > > > > Allow+putting+a+bound+on+memory+consumed+by+Incoming+requests > > > > > > > > The kip allows specifying a limit on the amount of memory allocated > for > > > > reading incoming requests into. This is useful for "sizing" a broker > > and > > > > avoiding OOMEs under heavy load (as actually happens occasionally at > > > > linkedin). > > > > > > > > I believe I've addressed most (all?) concerns brought up during the > > > > discussion. > > > > > > > > To the best of my understanding this vote is about the goal and > > > > public-facing changes related to the new proposed behavior, but as > for > > > > implementation, i have the code up here: > > > > > > > > https://github.com/radai-rosenblatt/kafka/tree/broker-memory > > > > -pool-with-muting > > > > > > > > and I've stress-tested it to work properly (meaning it chugs along > and > > > > throttles under loads that would DOS 10.0.1.0 code). > > > > > > > > I also believe that the primitives and "pattern"s introduced in this > > KIP > > > > (namely the notion of a buffer pool and retrieving from / releasing > to > > > said > > > > pool instead of allocating memory) are generally useful beyond the > > scope > > > of > > > > this KIP for both performance issues (allocating lots of short-lived > > > large > > > > buffers is a performance bottleneck) and other areas where memory > > limits > > > > are a problem (KIP-81) > > > > > > > > Thank you, > > > > > > > > Radai. > > > > > > > > > > -- Regards, Rajini
