Pongo Sapiens

Yes, and the server starts to lose TPS fairly quickly after a restart.

@unknown We are aware of the actual version, several people have stated that all releases 15w47c and earlier did not suffer from this bug. This bug report is EXPLICITLY about lag that started in 15w49a. If you are looking at lag that existed before 15w49a, then it is NOT this bug.
@unknown once typo-ed 15w49a as 15w47a, but apart from that it's been consistent.
So it is back at the head of the comments, the lag we are discussing here WAS NOT PRESENT in 15w47c, it STARTED in 15w49a

Please note that the only person that attached terrain generation to this bug was Steve Mortensen, I have not noticed any change in the behavior of terrain generation associated with the transition from 15w47c to 15w49a, which is what this bug report is explicitly about.
It is not about generalized server lag, it is about a huge dip in performance that started in 15w49a, and is directly related to pathfinding (which was also one of the major changes introduced in 15w49a). There is detailed analysis that demonstrates this in the comment chain.

It seems entirely possible that this bug could exacerbate the memory leak described in MC-100382 but given the age of the underlying problem in that bug, it seems unlikely to be the root cause.

Kumasasa: How is this a client issue? Everything reported here has been in relation to servers.

@Meri Diana
Please can we keep this bug on topic, despite the suggestions that it's some kind of dumping ground for random lag issues, it has mostly been very focussed. Meta-discussions don't belong here.

Thanks.

@Meri Diana
I disagree, the bug title is very specific: a change (or changes) introduced between 15w47c and 15w49a (the next release) caused massive lag. This actually significantly limits the scope of what can cause it. The bug is resolved when performance returns to the same level as 15w47c (and it's pre-cursors), or the devs explain why that won't happen. As it stands, 16w07a is the first version since 15w47c that I feel I can realistically play on, but it's still far from the performance of that earlier version.

@fienxjox - are you actually in contact with the devs? I believe that there is plenty of evidence in the comments for the devs to find the issue, there's profiling output, information on when then issue started and information on how to re-create the issue (make a world, spawn mobs, introduce path finding complexity, watch world stop).
Simple experiments suggest that mobs in a super flat world do not cause issues, but it is relatively easy to trigger issues, simply introduce barriers to free travel and motivation for the mobs to path find.

I'm fairly convinced that the command block issue being experienced by Liam is a separate issue that is likely being severely exacerbated by the primary issue in this bug.

My world has mobs and some redstone (no clocks but a chicken cooker that triggers intermittently) and suffers from lag.
I should note that the number of possible causes is SIGNIFICANTLY reduced by the fact that it was a change introduced post 15w47c, and hence a simple bisection should quickly identify the issue.

Many people on this thread, have provided detailed traces of likely causes of this issue. The original reporter was not using a modified world. Almost all the people I know that run servers have stuck with 15w47c because the later snapshots are unplayable. My primary server suffers from this lag and was a new world with 15w51c. People that submit watchdog timeout crash dumps get them closed as duplicates of an unrelated issue because the "watch dog timer is working as intended", hence you see no server crash logs.

There is a LOT of evidence in this thread that points at new mob AI and path finding, is this the only cause? We don't know, but the closer we get to release without this issue being addressed, the less likely it is that we'll find out.

In summary, trading anecdote for anecdote, I don't know of any instance of a post 15w47c server that doesn't suffer from this problem, and I have direct experience of many such servers of various histories. For more evidence, look at Etho's latest MC video on youtube where he has unexplained severe lag in his single player world.

We are at this point pretty sure that the major cause of lag is Mob AI. See earlier comments that point the finger firmly at "the pathfinding algorithm jams the game (from the first report)".

There may be a separate issue with command blocks, but this is unrelated.

This is not a duplicate of 63590, the POINT to the crash logs, the POINT to the watchdog is that something bad has happened. It is indeed the case that the server watchdog is "behaving as intended", it has generated a crashlog that Mojang can investigate to detect the REAL badness, if you then reject these crash logs as duplicates of an UNRELATED issue, you are actively working against the watchdog and preventing it from "working as intended".

I am getting massive lag on a 16-core server where I've given Java 8Gb of ram, while it is admittedly amplified terrain, there are only two of us playing, and it only needs one of us for it to start lagging. This is 15w51b. I am seeing the same console spam and getting watchdog crashes.

More generally no mobs/animals will climb steps in a north/south direction, they will actively seek a path that allows them to climb east/west and stall if they don't find one.

This is quite a difficult bug to replicate. I believe that it is caused by entities traveling across chunk boundaries while the world is being saved. I will attempt to test this in the latest snapshot this weekend. I have no reason to believe that it will have been fixed though.

I know that Minecraft records all sounds, not only in game sound and Mic, and that is the behavior I like. I don't think that this behavior is a bug...

I repeat I prefer recording all sounds to only recording in game sounds + mic.

I like the the current behavior.

I like the current behavior, it would be nice if you could select sounds sources, but I much prefer the current behavior to only having in game sounds and mic.

I believe that this is a feature request, not a bug.

That exactly replicates what I've tested so far. Thanks. I'm still playing with vertical chains as I thought I saw really weird behavior with items "leapfrogging" down the chain, but I cannot reproduce and it may have been what you described in your test 2.

Or should I open a new report for the sending issue?

Please re-open this as the sending bug is a show stopper.

I have a farm with a hopper minecart on a track which had stopped working, when I investigated I found there were 5 hopper minecarts on the track.
This is in 1.8.2-pre1