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So! I've made a giant mob spawner/killer up in the sky (y=220-235) over an ocean. Light level 1 spawn floors, 540 block area, 3 high, periodic water flush that pushes mobs into a pit, where fall damage kills them.

Problem is, mobs seems to rarely spawn inside, and endermen just vanish. I know they teleport on contact with water, but shouldn't they appear outside the trap on one of the catwalks? The floor they land on is ~40 blocks lower, and there's nothing else up that high.

So my question(s) are, why aren't more monsters spawning? I'll leave it running for half an hour, and there will be 2 spawns. Other times, I'll get 30 kills in 10 minutes. Any idea why?

Bonus round: Where the hell are the endermen going? Why do they hate my trap?

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A number of observations...if you're that high, you are more than 100 above the ocean, so all mobs should be spawning within your trap. The trap floor must not be half slabs, or similar, because you're getting spawning. Light levels must be reasonable, else you'd be getting zero spawn...Where are you waiting relative to your trap? If you're far from the trap (or too close) that could be causing you problems. Screen shots? –  John Jan 11 '13 at 18:21
    
Good point about how far away you wait; I think I had misinterpreted how the spawning range works. 24-32 blocks is the range they appear in and move around, correct? (Screens: i.imgur.com/zNRcx.png i.imgur.com/rhQaU.png Pretty dark, but then, it's kind of supposed to be, right?) –  ColdComfort Jan 11 '13 at 19:19
    
@ColdComfort I was hoping for a screen shot of your trap. Maybe some torches would help. Yes, mobs appear > 24, and move around < 32 (roughly). Does that affect things for you? –  John Jan 11 '13 at 20:29
    
Ha ha, that IS a screenshot of the trap, albeit from the inside. Probably not too clear. I tried standing closer, but it doesn't seem to make much difference, unfortunately... –  ColdComfort Jan 11 '13 at 20:58
    
I can't think of any problems with the spawning, and your trap should, in theory, work. However maybe there is a problem somewhere other than spawning. My mob trap kept getting clogged up by spiders at one point, because they will sometimes climb walls instead of dropping to where you need them to go. I would suggest lighting up the place and doing a full check to make sure it's all in order, or maybe getting a single mob to spawn, and then bait him through the whole trap to see if he gets through successfully. –  Ryan Jan 12 '13 at 0:55
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3 Answers

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Mob traps built close to the sky limit have been known to end up not being completely loaded by the engine in some situations. The way to test for this is to open the F3 screen and look at the 'lc' value. This number is the highest Y-level that is currently loaded. If it's lower than the top of your mob trap then you would need to add some blocks above the trap to force the entirety of it to be loaded. This youtube video shows how you can fix the problem if it happens

It's also worth mentioning that the high altitude mob traps are (all other factors being equal) always going to be at least somewhat slower than low altitude mob traps simply because there ends up being more blocks for the spawning algorithm to check

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lc value is 255, so that's higher than the top of my trap, so that should be fine. I would have thought that building it in the sky would REDUCE the number of blocks it would need to check, because most of them would be air, right? Am I missing something? –  ColdComfort Jan 12 '13 at 6:23
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Building in the sky doesn't decrease the number of blocks to check because air blocks are still blocks. See this overview of the spawning algorithm for more details. What building in the sky does do is make it much easier to prevent mobs spawning outside your trap. –  Steven Cunningham Jan 12 '13 at 7:07
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If there is a cave system under your mob trap (underground) the mobs will rarly spawn in your trap.

How much floor is there without water (mobs can't spawn in water)

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The water is on a timed piston circuit, probably about 30 seconds between flushes, which last about 10 seconds each. So, the whole trap floods every 30 seconds or so. (Does that affect spawning, or detection of spawnable areas?) I'm pretty sure I've lit up all the cave areas under the trap, and I'm 150 blocks above ground level, so stuff shouldn't be spawning outside of a 128 block radius from me, right? The only thing within 128 blocks is my trap! –  ColdComfort Jan 11 '13 at 18:21
    
try making the time between flushes bigger sethbling made on with 5 minute timer: youtube.com/… at 27 mins he shows it :D –  RustyMembers Jan 12 '13 at 9:58
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Yeah I was thinking the timing might affect it as well. If you think about it, every 30 seconds, water is flooding in. How long is the residual water around? 5, 10 seconds? You might be reducing the amount of time the area is available for spawning. Try wiring a manual shutoff for the flooding and see how your spawning goes.

Another thought...

Remember mobs spawn outside 32-40 (whatever it is) but do not move. You could try a shifting floor design. Gives you a larger area as you're not relying on the mobs falling into the water streams. This also works around the 32-40 range limit. It may also make a simpler trap. Just put water in the corners below the shifting floor with a sinkhole in the middle. This would then easily be stackable. Just put another spawning floor above it.

To be even more fancy, put some vines at the last layer and rig a piston at the bottom. Makes for a toggle items / items+XP farm. Helps to get those drops only from killing them by hand.

Also try expanding the spawning area too. Maybe a 2x2 or 3x3 chunk area? This would give you a 1024 or 2304 area. It's more setup but you have lots of empty space to work with.

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