[GHC] #14407: rts: Threads/caps affinity

31 Oct 2017

      #14407: rts: Threads/caps affinity
-------------------------------------+-------------------------------------
           Reporter:  pacak          |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Runtime        |           Version:  8.3
  System                             |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Currently GHC supports two kinds of threads with respect to thread
 migration - pinned to a specific capability and those it can migrate
 between any capabilities. For purposes of achieving lower latency in
 Haskell applications it would be nice to have something in between -
 threads GHC rts can migrate but within a certain subset of capabilities
 only.

 I'm developing a program that contains several kinds of threads - those
 that do little work and sensitive to latency and those that can spend more
 CPU time and less latency sensitive. I looked into several cases of
 increased latency in those sensitive threads (using GHC eventlog) and in
 all cases sensitive threads were waiting for non-sensitive threads to
 finish working. I was able to reduce worst case latency by factor of 10 by
 pinning all the threads in the program to specific capability but manually
 distributing threads (60+ of them) between capabilities (several different
 machines with different numbers of cores available) seems very fragile.
 World stopping GC is still a problem but at least in my case is much less
 frequently so.

 I have a patch for rts that implements this proposal

 {{{#!hs

 {- | 'setThreadAffinity' limits RTS ability to migrate thread to
 capabilities with numbers that matches set bits of affinity mask, thus
 mask of `0b101` (5) will allow RTS to migrate this thread to caps
 0 (64, 128, ..) and 3 (64 + 3 = 67, 128 + 3 = 131, ...).

 Setting all bits to 0 or 1 will disable the restriction.
 -}
 setThreadAffinity :: ThreadId -> Int -> IO ()
 }}}

 This allows to define up to 64 distinct groups and allow user to break
 down their threads into bigger number of potentially intersecting groups
 by specifying things like capability 0 does latency sensitive things, caps
 1..5 - less  sensitive things, caps 5-7 bulk things.

 Sample program using this API

 {{{#!hs
 {-# LANGUAGE LambdaCase #-}

 import Data.Time
 import Control.Monad
 import Control.Concurrent
 import System.Environment (getArgs)
 import GHC.Conc

 wastetime :: Bool -> IO ()
 wastetime affine = do
     tid <- forkIO $ do

         myThreadId >>= \tid -> labelThread tid "timewaster"

         forever $ do
             when (sum [1..1000000] < (0 :: Integer)) $
                 print "impossible"
             threadDelay 100
             yield
     when affine $ setThreadAffinity tid (255 - 2)

 client :: Bool -> IO ()
 client affine = do
     myThreadId >>= \tid -> labelThread tid "client"
     when affine $ myThreadId >>= \tid -> setThreadAffinity tid 2
     before <- getCurrentTime
     replicateM_ 10 $ do
         threadDelay 10000
     after <- getCurrentTime
     print $ after `diffUTCTime` before

 startClient :: Bool -> IO ()
 startClient = {- replicateM_ 10 . -} client

 main :: IO ()
 main = do

     getArgs >>= \case
         [wno's, aff's] -> do
             let wno = read wno's
                 aff = read aff's
             putStrLn $ unwords ["Affinity:", show aff, "Timewasters:",
 show wno]
             replicateM_ wno (wastetime aff)
             startClient aff
         _ -> putStrLn "Usage: <progname> <number of time wasters> <enable
 affinity: True/False>"

 }}}

 Compiled with -threaded and running with rts -N8 on 6 core (12 threads)
 machine.

 Results are noisy but repeatable

 {{{
 Affinity: False Timewasters: 24
 0.42482036s
 }}}

 {{{
 Affinity: True Timewasters: 24
 0.111743474s
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14407>
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