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IRC log for #minetest-dev, 2015-05-26

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Time Nick Message
00:18 paramat joined #minetest-dev
00:19 paramat i decided to go ahead with #2720 will push very soon. note i am changing humidity octaves to 3 and persistence to 0.5 for fewer tiny jungle biome bubbles
00:19 ShadowBot https://github.com/minetest/minetest/issues/2720 -- Mgv6: Enable snowbiomes by default. Double biome noise spread. by paramat
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01:06 paramat now pushing
01:11 paramat complete
01:19 hmmmm see?  biome bubbles are why I prefer minecraft's biome shape
01:21 paramat i don't mind a few, an occasional one is cute. lower octaves and persistence helps to reduce their number
01:22 hmmmm yeah that also makes biomes... square
01:22 hmmmm rather, that predictable off-center blob shape
01:22 hmmmm it's a natural side effect of easeCurve + bilinear interpolation
01:23 paramat that's very noticeable with < 3 octaves
01:23 hmmmm see
01:23 hmmmm this is why I wanted to check out cubic interpolation
01:24 sofar paramat: which mapgen has your changes? v6 ?
01:26 paramat yes
01:27 paramat it has kick-ass big jungles now
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01:29 sofar it's creating tunnels underwater?
01:34 paramat that's probably a previous bug
01:34 paramat um.. 'characteristic'
01:35 sofar lol
01:35 paramat i've noticed v6 jungles get very thick and dark sometimes, now they're larger they're a real adventure, instead of pathetic 60 node-wide patches
01:35 * sofar cant find snow
01:36 paramat might need to travel 1kn
01:36 paramat or more
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01:48 paramat you need to start a new world and make sure you don't have 'mgv6_spflags = nosnowbiomes' in .conf. the underwater tunnels are i think air-dungeons forming
01:49 sofar oh, that's on by default
01:51 sofar huh
01:51 sofar something put it back, too
01:54 sofar ugh
01:54 sofar something keeps putting the "nosnowbiomes" param back in there
01:54 sofar paramat: ?
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01:59 sofar paramat: can't find snow... had a hard time removing nosnowbiomes, and now I get this:
01:59 sofar http://i.imgur.com/BVlwbOX.png
02:00 sofar maybe my minetest_game is old
02:00 * sofar pulls
02:01 sofar ahh that did the trick
02:01 sofar paramat: looks great!
02:05 sofar http://i.imgur.com/LAb2xuT.jpg
02:07 paramat oh yes, need latest MTGame *=/
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05:51 cd2 heyo :D
05:52 est31 hi
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15:09 RealBadAngel https://github.com/RealBadAngel/minetest/tree/minimap3
15:09 RealBadAngel if somebody wants to try minimap progress
15:10 RealBadAngel only config option is minimap_shape_round = true/false
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15:30 hmmmm hrmm let's see this minimap
15:31 hmmmm a thread
15:31 hmmmm there's a new thread!??!
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15:32 hmmmm hrmm I need to take a better look at this but my hope was that this could be done without introducing more client-side contention
15:34 hmmmm there's no locking done at all?
15:35 hmmmm the only way this isn't crashing is through pure luck
15:35 hmmmm also wtf is the "minimap mode"?
15:37 celeron55 umm
15:37 celeron55 i started reading this too and noticed the same thing
15:37 celeron55 there is no locking at all when reading the client's map from a thread
15:39 hmmmm okay, the particulars of the code are a little sloppy but nothing that can't be fixed
15:39 hmmmm is having a separate minimap thread the optimal approach, though?
15:39 hmmmm what if we throw this into the client step
15:39 hmmmm also i personally feel like 10ms is a bit too short of an update time
15:40 celeron55 do you mean 100ms
15:40 hmmmm why?
15:40 celeron55 what's how long it sleeps
15:40 hmmmm it says sleep_ms(10); in minimap.cpp line 73
15:40 celeron55 what's*
15:41 celeron55 https://github.com/RealBadAngel/minetest/blob/minimap3/src/minimap.cpp#L74
15:41 celeron55 i wonder what version you are reading
15:41 hmmmm an older version
15:41 hmmmm he changed it in the commit at HEAD
15:42 hmmmm woops
15:42 hmmmm i dunno, celeron, what do you think about having the minimap in a separate thread?
15:43 celeron55 i think if this was run in the render thread the x*z loop would have to be paused between frames and done in multiple parts that way; i feel it's not going to be (and shouldn't need to be) fast enough to be run within a single frame
15:43 celeron55 if it would be fast enough to run within a single frame, then it should be split so that we can have larger map areas
15:43 hmmmm why, because the nodes could have updates throughout the render thread?
15:44 celeron55 no but because it's a rather heavy operation to find the ground level for tens of thousands or more positions
15:45 hmmmm oh, that is true
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15:45 celeron55 frametime jitter is a terrible and embarrassing thing to have, especially if it was due to a thing like this
15:45 hmmmm didn't think of that
15:47 celeron55 i
15:48 celeron55 i'm also not sure at all that these irrlicht texture operations that are being done in the thread are actually thread-safe
15:48 celeron55 the image ones are re-entrant as far as i know (i don't know if they are specified to be, but they likely are), but when you create an image from a texture, that might go on scarier territory
15:48 celeron55 dunno
15:49 hmmmm good to note
15:50 celeron55 it might also be platform dependent or whatever; i'm pretty sure irrlicht was never designed to run at all in multiple threads so it's up to us to figure out what can be done and what cannot
15:50 hmmmm another thing
15:51 hmmmm all of the ground levels are recalculated every getMap() call
15:51 hmmmm since users are most likely walking from one place to another, shouldn't there be a heightmap cache
15:54 celeron55 probably, altough i don't think the performance should be dependent on it
15:54 RealBadAngel hmmm, thread and mapper share their data and trigger events between them, so no crashing possible, its no luck
15:55 celeron55 oh god
15:55 RealBadAngel also whole scan have to be completed each time, theres no fixed ground level for surface scanner
15:56 celeron55 i'm 100% sure that kind of synchronization has subtle bugs in it
15:56 RealBadAngel had, ive eliminated them ;)
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15:56 RealBadAngel simply, if scan is not complete draw cannot pick another frame
15:57 celeron55 i wonder if minetest runs cleanly enough in helgrind or drd for checking things like this
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15:57 celeron55 it might not
15:57 RealBadAngel anyway i may change it, never touched threads before
16:00 rubenwardy How to use minimap?
16:00 RealBadAngel press f9
16:00 rubenwardy awesome
16:00 RealBadAngel it looks better with shaders ;)
16:01 rubenwardy is that not in minimap3?
16:01 RealBadAngel it is there
16:02 RealBadAngel also in config: minimap_shape_round = true/false
16:04 rubenwardy shape_round also affects the behaviour when rotating, it seems
16:05 RealBadAngel yes, shadows are dynamic then
16:05 RealBadAngel and theres no player marker
16:06 rubenwardy quite weird behaviour underground
16:06 rubenwardy no player marker, and the thing spins, which is cool
16:07 rubenwardy I guess radar is recommended for underground
16:07 RealBadAngel exactly
16:07 RealBadAngel it scans for air nodes and count them, so more air, px on the map is more green
16:08 RealBadAngel i found that works pretty good when digging tunnels
16:08 rubenwardy I'd like to see a player marker on the circle map - just a dot - it's hard to judge exactly where you are
16:08 rubenwardy That was probably badly worked
16:08 RealBadAngel always in the middle
16:08 rubenwardy * worded
16:09 rubenwardy Yeah, but it's human eyes are weird with judges halfs
16:09 rubenwardy It's not required
16:09 RealBadAngel also do notice that marker occupies the middle, it could be hard to see nodes you are placing
16:09 VanessaE player marker on the circular map can be done - as soon as RealBadAngel is able to again use the texture I created ;)
16:10 rubenwardy A pixel
16:10 rubenwardy Yeah, digging is quite cool
16:10 VanessaE a 5x5 crosshair with an empty center pixel. maybe.
16:10 RealBadAngel you can do that with overlay texture
16:11 RealBadAngel so its up for texture pack
16:11 hmmmm RealBadAngel, what do you mean by this   [11:56 AM] <RealBadAngel> simply, if scan is not complete draw cannot pick another frame
16:12 RealBadAngel it locks itself, data cannot be grabbed if scan is not done
16:13 rubenwardy Sounds logical. Like a buffer locking, if it's not finished the reader just keeps reading from the old.
16:13 rubenwardy * double buffer
16:13 RealBadAngel scanner work on images
16:13 hmmmm i don't see where this happens
16:13 RealBadAngel not threaded part uses textures
16:14 RealBadAngel first of all, getMap gets copies of parameters so changes during scan doesnt affect scanner
16:14 hmmmm okay, that's fine
16:15 RealBadAngel when its done it changes image_grabbed flag
16:15 RealBadAngel so draw code can pick another frame
16:15 RealBadAngel if theres no new frame ready, draw uses old one
16:15 rubenwardy Would it be possible to hide jolting (ie, on mine when walking the map only moves every 1/10 or a second) by moving where the texture is drawn, if the minimap takes too long?
16:16 hmmmm hrmm
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16:16 rubenwardy I probably misunderstand how it's done.
16:16 hmmmm I am not understanding how this works
16:16 RealBadAngel on my box i do get minimap fps like half of overall fps
16:17 RealBadAngel in 256x256 mode
16:17 RealBadAngel in others scanner is faster than 60fps
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16:17 hmmmm what thread is Mappger::getMinimapTexture() called in?
16:18 RealBadAngel main
16:18 rubenwardy Is the minimap constantly updating, and the jolting is when it finishes a tick? Or does it sleep?
16:18 rubenwardy Maybe I should just read the code, lol. :P
16:18 hmmmm somehow I wonder if uploading a texture every 100 ms is a good thing
16:19 RealBadAngel huh its done everywhere
16:19 RealBadAngel each frame
16:19 RealBadAngel rendering to textures for example
16:19 hmmmm so getMinimapTexture only swaps the texture if image_grabbed is false
16:20 hmmmm those are cached
16:20 hmmmm image_grabbed is sort of misleading
16:20 hmmmm more like... "minimap_texture_invalidated"
16:20 RealBadAngel could be named better ;)
16:21 RealBadAngel also about thread, only getMap will remain there
16:21 RealBadAngel i already moved getColorFromId from runtime
16:22 RealBadAngel so thread will work only on open images and will create just one new image each run
16:22 hmmmm so I don't understand how the whole image_grabbed thing prevents a race condition
16:25 RealBadAngel if false, it cannot start new scan
16:25 RealBadAngel ooops
16:25 RealBadAngel hold on a sec, i will make a diagram
16:26 hmmmm it doesn't matter, it doesn't need a diagram
16:26 hmmmm the fact remains that any other thread can access map and not respect this image_grabbed variable at all
16:26 hmmmm unless i'm totally mistaken, you're depending upon the order in which things happen inside of the render thread
16:27 celeron55 and likely the duration of things too
16:28 hmmmm this is remarkably fragile code...
16:28 hmmmm RealBadAngel, even if I'm totally wrong, what's the cost of adding uncontended locks around map accesses?
16:28 hmmmm 70 nanoseconds?
16:29 hmmmm the texture locks are a bit more subtle, however...
16:31 RealBadAngel scanner cant make new scan if old one is not used, renderer cannot pick incomplete scan for texture, wheres depending on duration there?
16:31 RealBadAngel theres no such thing in whole code
16:33 RealBadAngel hmmmm, and i dont depend on order of anything
16:33 RealBadAngel if renderer doesnt have new frame it will try next time
16:33 RealBadAngel otherwise old texture is used
16:34 hmmmm I'm talking about map access here
16:34 RealBadAngel here im not sure
16:34 RealBadAngel i do copy blocks to vmanip
16:35 RealBadAngel should i do that under locking?
16:35 hmmmm YES.....
16:35 RealBadAngel so why mapblock mesh updates doesnt do that?
16:35 hmmmm i don't know
16:35 hmmmm but it should
16:35 celeron55 they copy their content to vmanips in the client thread
16:35 celeron55 and then give the vmanips to the mesh thread
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16:36 celeron55 you can do the same thing here if you want and if it's fast enough
16:36 hmmmm in any case, image_grab needs to be volatile
16:38 RealBadAngel celeron55, i can move copying to main, no problem
16:41 RealBadAngel hmmmm,  that variable is the lock in fact, i can mention bout it in name
16:42 RealBadAngel and i will do cleanin now, theres lotsa dead code in that branch
16:43 RealBadAngel btw, have you guys checked the performance with minimap?
16:43 RealBadAngel how it actually works for you?
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16:52 hmmmm also Mapper::setPos() has a potential race condition
16:52 RealBadAngel doesnt have
16:52 RealBadAngel getMap works on copies of parameters
16:53 RealBadAngel on the next run it will get updated pos
16:53 RealBadAngel same applies to mode
16:54 RealBadAngel you can check that when flying fast and 256x256 map
16:54 RealBadAngel scanner stays a little behind the player
16:55 RealBadAngel not much but thats visible (at least on my box)
16:56 hmmmm they might not be updated before they're copied
16:56 hmmmm this is why they need to be volatile
16:57 VanessaE actually it works just fine in practice.
16:57 hmmmm yes, but we are not concerned about "in practice"
16:57 hmmmm we need to worry about what can happen in theory, because what can happen will happen
16:57 VanessaE fair enough
16:59 RealBadAngel if its locked it cant do anything
16:59 RealBadAngel scanner cannot scan, renderer cannot pick incomplete data
16:59 hmmmm RealBadAngel, yes it can
16:59 hmmmm do you not realize that compilers sometimes reorder operations?
17:00 RealBadAngel how they can reorder variable value?
17:00 RealBadAngel its the condition for all the actions here
17:00 hmmmm read about this http://preshing.com/20120515/memory-reordering-caught-in-the-act/
17:01 RealBadAngel ok
17:01 hmmmm what you think is thread safe really is not
17:01 hmmmm you need to worry about not only atomicity, but also memory access reordering
17:02 hmmmm so you have in one thread
17:02 hmmmm texture = mapper.getMinimapTexture(); .... mapper.setPos(newpos);
17:03 hmmmm image_grabbed = true, so the thread starts to call getMap()
17:03 hmmmm and it copies over the parameters
17:03 hmmmm well it just so happens that newpos didn't completely finish writing to data->pos, it's only half written
17:04 hmmmm so your old position was (200, 0, 400) or something and the new position is (4000, 30, 12)
17:04 hmmmm it only had time to write 2 of the 4 16-bit numbers within that access
17:04 hmmmm so it ends up being, say... (200, 30, 12)
17:05 hmmmm and the same thing with mode too
17:05 hmmmm assuming that accesses to variables of type bool are atomic on our supported platforms is pretty fair
17:05 RealBadAngel hold on a sec
17:05 hmmmm but a v3s16 is NOT ATOMIC on any of our platforms
17:05 hmmmm it simply isn't
17:06 RealBadAngel only thing that can trigger something here is that flag
17:06 RealBadAngel before it gets written nothin can happen
17:06 hmmmm I just explained in great detail how this race condition happens
17:07 hmmmm and then you handwave it away with "but the flag is there!"
17:07 RealBadAngel im just trying to understand hows that possible
17:07 hmmmm read what i wrote again, carefully
17:13 RealBadAngel ok, so it may happen. whats your suggestion to prevent that?
17:13 hmmmm locking
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17:28 RealBadAngel hmmmm, any examples?
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17:47 RealBadAngel hmmmm, from what i read it seems like converting v3s16 to three atomic ints would do the trick and have lock-free code
18:07 Krock hmmmm, Is it thought to be changed like this? https://github.com/SmallJoker/yappy/commit/1f0b
18:08 Krock (Talking about the memory efficiency improvements)
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