Re: Why do remaining HECs busy-wait during unsafe-FFI calls?
When an unsafe call is made, the OS thread currently running on the HEC makes the call without releasing the HEC. If the main thread was on the run queue of the HEC making the foreign unsafe call when the foreign call was made, then no other HECs will pick up the main thread. Hence the two sleep calls in your program happen sequentially instead of concurrently. I'm not completely sure what is causing the busy wait, but here is one guess: when a GC is triggered on one HEC, it signals to all the other HECs to stop the mutator and run the collection. This waiting may be a busy wait, because the wait is typically brief. If this is true, then since one thread is off in a unsafe foreign call, there is one HEC that refuses to start the GC and all the other HECs are busy-waiting for the signal. The GC could be triggered by a period of inactivity. Again, this is just a guess - you might try to verify this by turning off the periodic triggering of GC and checking whether the start GC barrier is a busy-wait. On Mon, May 6, 2013 at 7:29 AM, Herbert Valerio Riedel h...@gnu.org wrote: Hello, Recently, I stumbled over E.Z.Yang's Safety first: FFI and threading[1] post and then while experimenting with unsafe-imported FFI functions I've noticed a somewhat surprising behaviour: Consider the following contrived program: --8---cut here---start-8--- import Foreign.C import Control.Concurrent import Control.Monad import Data.Time.Clock.POSIX (getPOSIXTime) foreign import ccall unsafe unistd.h sleep c_sleep_unsafe :: CUInt - IO CUInt main :: IO () main = do putStrLnTime main started _ - forkIO (sleepLoop 10 putStrLnTime sleepLoop finished) yield putStrLnTime after forkIO threadDelay (11*1000*1000) -- 11 seconds putStrLnTime end of main where putStrLnTime s = do t - getPOSIXTime putStrLn $ init (show t) ++ \t ++ s sleepLoop n = do n' - c_sleep_unsafe n unless (n' == 0) $ do putStrLnTime c_sleep_unsafe got interrupted sleepLoop n' --8---cut here---end---8--- When compiled with GHC-7.6.3/linux/amd64 with -O2 -threaded and executed with +RTS -N4, the following output is emitted: 1367838802.137419 main started 1367838812.137727 after forkIO 1367838812.137783 sleepLoop finished 1367838823.148733 end of main which shows that the forkIO of the unsafe ccall effectively blocks the main thread; Moreover, when looking at the process table, I saw that 3 threads were occupying 100% CPU time each for 10 seconds until the 'after forkIO' was emitted. So what is happening here exactly, why do the 3 remaining HECs busy-wait during that FFI call instead of continuing the execution of the main thread? Do *all* foreign unsafe ccalls (even short ones) cause N-1 HECs to spend time in some kind of busy looping? [1]: http://blog.ezyang.com/2010/07/safety-first-ffi-and-threading/ Cheers, hvr ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Cloud Haskell and network latency issues with -threaded
Hi Edward, I did two things to improve latency for my application: (1) rework the IO manager and (2) stabilize the work pushing. (1) seems like a big win and we are almost done with the work on that part. It is less clear whether (2) will generally help much. It helped me when I developed it against 7.4.1, but it doesn't seem to have much impact on HEAD on the few measurements I did. The idea of (2) was to keep running averages of the run queue length of each capability, then push work when these running averages get too out-of-balance. The desired effect (which seems to work on my particular application) is to avoid cases in which threads are pushed back and forth among cores, which may make cache usage worse. You can see my patch here: https://github.com/AndreasVoellmy/ghc-arv/commits/push-work-exchange-squashed . -Andi On Fri, Feb 8, 2013 at 12:10 AM, Edward Z. Yang ezy...@mit.edu wrote: Hey folks, The latency changes sound relevant to some work on the scheduler I'm doing; is there a place I can see the changes? Thanks, Edward Excerpts from Simon Peyton-Jones's message of Wed Feb 06 10:10:10 -0800 2013: I (with help from Kazu and helpful comments from Bryan and Johan) have nearly completed an overhaul to the IO manager based on my observations and we are in the final stages of getting it into GHC This is really helpful. Thank you very much Andreas, Kazu, Bryan, Johan. Simon From: parallel-hask...@googlegroups.com [mailto: parallel-hask...@googlegroups.com] On Behalf Of Andreas Voellmy Sent: 06 February 2013 14:28 To: watson.timo...@gmail.com Cc: kosti...@gmail.com; parallel-haskell; glasgow-haskell-users@haskell.org Subject: Re: Cloud Haskell and network latency issues with -threaded Hi all, I haven't followed the conversations around CloudHaskell closely, but I noticed the discussion around latency using the threaded runtime system, and I thought I'd jump in here. I've been developing a server in Haskell that serves hundreds to thousands of clients over very long-lived TCP sockets. I also had latency problems with GHC. For example, with 100 clients I had a 10 ms (millisecond) latency and with 500 clients I had a 29ms latency. I looked into the problem and found that some bottlenecks in the threaded IO manager were the cause. I made some hacks there and got the latency for 100 and 500 clients down to under 0.2 ms. I (with help from Kazu and helpful comments from Bryan and Johan) have nearly completed an overhaul to the IO manager based on my observations and we are in the final stages of getting it into GHC. Hopefully our work will also fix the latency issues in CloudHaskell programs :) It would be very helpful if someone has some benchmark CloudHaskell applications and workloads to test with. Does anyone have these handy? Cheers, Andi On Wed, Feb 6, 2013 at 9:09 AM, Tim Watson watson.timo...@gmail.com mailto:watson.timo...@gmail.com wrote: Hi Kostirya, I'm putting the parallel-haskell and ghc-users lists on cc, just in case other (better informed) folks want to chip in here. First of all, I'm assuming you're talking about network latency when compiling with -threaded - if not I apologise for misunderstanding! There is apparently an outstanding network latency issue when compiling with -threaded, but according to a conversation I had with the other developers on #haskell-distributed, this is not something that's specific to Cloud Haskell. It is something to do with the threaded runtime system, so would need to be solved for GHC (or is it just the Network package!?) in general. Writing up a simple C program and equivalent socket use in Haskell and comparing the latency using -threaded will show this up. See the latency section in http://haskell-distributed.github.com/wiki/networktransport.html for some more details. According to that, there *are* some things we might be able to do, but the 20% latency isn't going to change significantly on the face of things. We have an open ticket to look into this ( https://cloud-haskell.atlassian.net/browse/NTTCP-4) and at some point we'll try and put together the sample programs in a github repository (if that's not already done - I might've missed previous spikes done by Edsko or others) and investigate further. One of the other (more experienced!) devs might be able to chip in and proffer a better explanation. Cheers, Tim On 6 Feb 2013, at 13:27, kosti...@gmail.commailto:kosti...@gmail.com wrote: Haven't you had a necessity to launch Haskell in no-threaded mode during the intense network data exchange? I am getting the double performance penalty in threaded mode. But I must use threaded mode because epoll and kevent are available in the threaded mode only. [snip] среда, 6 февраля 2013 г., 12:33:36 UTC+2 пользователь Tim Watson написал: Hello all, It's been a busy week for Cloud
Re: Cloud Haskell and network latency issues with -threaded
On Fri, Feb 8, 2013 at 12:30 AM, Edward Z. Yang ezy...@mit.edu wrote: OK. I think it is high priority for us to get some latency benchmarks into nofib so that GHC devs (including me) can start measuring changes off them. I know Edsko has some benchmarks here: http://www.edsko.net/2013/02/06/performance-problems-with-threaded/ but they depend on network which makes it a little difficult to move into nofib. I'm working on other scheduler changes that may help you guys out; we should keep each other updated. That would be great :) I noticed your patch also incorporates the make yield actually work patch; do you think the improvement in 7.4.1 was due to that specific change? Actually, I believe that patch is irrelevant to the scheduler change and probably should not be in there, strictly speaking. I actually needed that patch for the IO manager revisions to work properly. (Have you instrumented the run queues and checked how your patch changes the distribution of jobs over your runtime?) I didn't do this very rigorously, but I think I added some print statements in the scheduler and I looked at some eventlogs in threadscope to see that threads work pushing slows down after a while. I had planned to write a script to analyze an event log file to extract these stats, but I never got around to it. -Andi Somewhat unrelatedly, if you have some good latency tests already, it may be worth a try compiling your copy of GHC -fno-omit-yields, so that forced context switches get serviced more predictably. Cheers, Edward Excerpts from Andreas Voellmy's message of Thu Feb 07 21:20:25 -0800 2013: Hi Edward, I did two things to improve latency for my application: (1) rework the IO manager and (2) stabilize the work pushing. (1) seems like a big win and we are almost done with the work on that part. It is less clear whether (2) will generally help much. It helped me when I developed it against 7.4.1, but it doesn't seem to have much impact on HEAD on the few measurements I did. The idea of (2) was to keep running averages of the run queue length of each capability, then push work when these running averages get too out-of-balance. The desired effect (which seems to work on my particular application) is to avoid cases in which threads are pushed back and forth among cores, which may make cache usage worse. You can see my patch here: https://github.com/AndreasVoellmy/ghc-arv/commits/push-work-exchange-squashed . -Andi On Fri, Feb 8, 2013 at 12:10 AM, Edward Z. Yang ezy...@mit.edu wrote: Hey folks, The latency changes sound relevant to some work on the scheduler I'm doing; is there a place I can see the changes? Thanks, Edward Excerpts from Simon Peyton-Jones's message of Wed Feb 06 10:10:10 -0800 2013: I (with help from Kazu and helpful comments from Bryan and Johan) have nearly completed an overhaul to the IO manager based on my observations and we are in the final stages of getting it into GHC This is really helpful. Thank you very much Andreas, Kazu, Bryan, Johan. Simon From: parallel-hask...@googlegroups.com [mailto: parallel-hask...@googlegroups.com] On Behalf Of Andreas Voellmy Sent: 06 February 2013 14:28 To: watson.timo...@gmail.com Cc: kosti...@gmail.com; parallel-haskell; glasgow-haskell-users@haskell.org Subject: Re: Cloud Haskell and network latency issues with -threaded Hi all, I haven't followed the conversations around CloudHaskell closely, but I noticed the discussion around latency using the threaded runtime system, and I thought I'd jump in here. I've been developing a server in Haskell that serves hundreds to thousands of clients over very long-lived TCP sockets. I also had latency problems with GHC. For example, with 100 clients I had a 10 ms (millisecond) latency and with 500 clients I had a 29ms latency. I looked into the problem and found that some bottlenecks in the threaded IO manager were the cause. I made some hacks there and got the latency for 100 and 500 clients down to under 0.2 ms. I (with help from Kazu and helpful comments from Bryan and Johan) have nearly completed an overhaul to the IO manager based on my observations and we are in the final stages of getting it into GHC. Hopefully our work will also fix the latency issues in CloudHaskell programs :) It would be very helpful if someone has some benchmark CloudHaskell applications and workloads to test with. Does anyone have these handy? Cheers, Andi On Wed, Feb 6, 2013 at 9:09 AM, Tim Watson watson.timo...@gmail.com mailto:watson.timo...@gmail.com wrote: Hi Kostirya, I'm putting the parallel-haskell and ghc-users lists on cc, just in case other (better informed) folks want to chip in here. First of all, I'm assuming you're talking about
Re: Cloud Haskell and network latency issues with -threaded
Hi Edsko, Can you explain the figure linked to on that page a bit? E.g. how should the axes be labelled? On Wed, Feb 6, 2013 at 9:33 AM, Edsko de Vries ed...@well-typed.com wrote: Hi, Just for clarity's sake (as the author of that Latency section that Tim referred to): I have addressed all of the issues listed there in Network.Transport.TCP, with the exception of the first (the -threaded issue). As Tim points out, this is not a Cloud Haskell specific issue; I have written this up as a short blog post at http://www.edsko.net/2013/02/06/performance-problems-with-threaded . Edsko On Wednesday, 6 February 2013 14:09:22 UTC, Tim Watson wrote: Hi Kostirya, I'm putting the parallel-haskell and ghc-users lists on cc, just in case other (better informed) folks want to chip in here. First of all, I'm assuming you're talking about network latency when compiling with -threaded - if not I apologise for misunderstanding! There is apparently an outstanding network latency issue when compiling with -threaded, but according to a conversation I had with the other developers on #haskell-distributed, this is not something that's specific to Cloud Haskell. It is something to do with the threaded runtime system, so would need to be solved for GHC (or is it just the Network package!?) in general. Writing up a simple C program and equivalent socket use in Haskell and comparing the latency using -threaded will show this up. See the latency section in http://haskell-distributed.**github.com/wiki/* *networktransport.htmlhttp://haskell-distributed.github.com/wiki/networktransport.htmlfor some more details. According to that, there *are* some things we might be able to do, but the 20% latency isn't going to change significantly on the face of things. We have an open ticket to look into this (https://cloud-haskell.** atlassian.net/browse/NTTCP-4https://cloud-haskell.atlassian.net/browse/NTTCP-4) and at some point we'll try and put together the sample programs in a github repository (if that's not already done - I might've missed previous spikes done by Edsko or others) and investigate further. One of the other (more experienced!) devs might be able to chip in and proffer a better explanation. Cheers, Tim On 6 Feb 2013, at 13:27, kost...@gmail.com wrote: Haven't you had a necessity to launch Haskell in no-threaded mode during the intense network data exchange? I am getting the double performance penalty in threaded mode. But I must use threaded mode because epoll and kevent are available in the threaded mode only. [snip] среда, 6 февраля 2013 г., 12:33:36 UTC+2 пользователь Tim Watson написал: Hello all, It's been a busy week for Cloud Haskell and I wanted to share a few of our news items with you all. Firstly, we have a new home page at http://haskell-distributed.** github.com http://haskell-distributed.github.com, into which most of the documentation and wiki pages have been merged. Making sassy looking websites is not really my bag, so I'm very grateful to the various author's whose Creative Commons licensed designs and layouts made it easy to put together. We've already had some pull requests to fix minor problems on the site, so thanks very much to those who've contributed already! As well as the new site, you will find a few of us hanging out on the #haskell-distributed channel on freenode. Please do come along and join in the conversation. We also recently split up the distributed-process project into separate git repositories, one for each component that makes up Cloud Haskell. This was done partly for administrative purposes and partly because we're in the process of setting up CI builds for all the projects. Finally, we've moved from Github's issue tracker to a hosted Jira/Bamboo setup at https://cloud-haskell.**atlassian.nethttps://cloud-haskell.atlassian.net- pull requests are naturally still welcome via Github! Although you can browse issues freely without logging in, you will need to provide an email address and get an account in order to submit new ones. If you have any difficulties logging in, please don't hesitate to contact me directly, via this forum or the cloud-haskell-developers mailing list (on google groups). As always, we'd be delighted to hear any feedback! Cheers, Tim -- You received this message because you are subscribed to the Google Groups parallel-haskell group. To unsubscribe from this group and stop receiving emails from it, send an email to parallel-haskell+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out. ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
