小编典典

在Haskell中使用GNU / Linux系统调用`splice`进行零拷贝Socket到Socket的数据传输

linux

更新:尼莫先生的回答帮助解决了这个问题! 下面的代码包含修复程序!请参见下面的nb Falsenb True呼叫。

还有一个称为的新Haskell软件包splice(具有最著名的套接字到套接字数据传输循环的特定于操作系统的可移植实现)

我有以下(Haskell)代码:

#ifdef LINUX_SPLICE
#include <fcntl.h>
{-# LANGUAGE CPP #-}
{-# LANGUAGE ForeignFunctionInterface #-}
#endif

module Network.Socket.Splice (
    Length
  , zeroCopy
  , splice
#ifdef LINUX_SPLICE
  , c_splice
#endif
  ) where

import Data.Word
import Foreign.Ptr

import Network.Socket
import Control.Monad
import Control.Exception
import System.Posix.Types
import System.Posix.IO

#ifdef LINUX_SPLICE
import Data.Int
import Data.Bits
import Unsafe.Coerce
import Foreign.C.Types
import Foreign.C.Error
import System.Posix.Internals
#else
import System.IO
import Foreign.Marshal.Alloc
#endif


zeroCopy :: Bool
zeroCopy =
#ifdef LINUX_SPLICE
  True
#else
  False
#endif


type Length =
#ifdef LINUX_SPLICE
  (#type size_t)
#else
  Int
#endif


-- | The 'splice' function pipes data from
--   one socket to another in a loop.
--   On Linux this happens in kernel space with
--   zero copying between kernel and user spaces.
--   On other operating systems, a portable
--   implementation utilizes a user space buffer
--   allocated with 'mallocBytes'; 'hGetBufSome'
--   and 'hPut' are then used to avoid repeated 
--   tiny allocations as would happen with 'recv'
--   'sendAll' calls from the 'bytestring' package.
splice :: Length -> Socket -> Socket -> IO ()
splice l (MkSocket x _ _ _ _) (MkSocket y _ _ _ _) = do

  let e  = error "splice ended"

#ifdef LINUX_SPLICE

  (r,w) <- createPipe
  print ('+',r,w)
  let s  = Fd x -- source
  let t  = Fd y -- target
  let c  = throwErrnoIfMinus1 "Network.Socket.Splice.splice"
  let u  = unsafeCoerce :: (#type ssize_t) -> (#type size_t)
  let fs = sPLICE_F_MOVE .|. sPLICE_F_MORE
  let nb v = do setNonBlockingFD x v
                setNonBlockingFD y v
  nb False
  finally
    (forever $ do 
       b <- c $ c_splice s nullPtr w nullPtr    l  fs
       if b > 0
         then   c_splice r nullPtr t nullPtr (u b) fs)
         else   e
    (do closeFd r
        closeFd w
        nb True
        print ('-',r,w))

#else

  -- ..

#endif


#ifdef LINUX_SPLICE
-- SPLICE

-- fcntl.h
-- ssize_t splice(
--   int          fd_in,
--   loff_t*      off_in,
--   int          fd_out,
--   loff_t*      off_out,
--   size_t       len,
--   unsigned int flags
-- );

foreign import ccall "splice"
  c_splice
  :: Fd
  -> Ptr (#type loff_t)
  -> Fd
  -> Ptr (#type loff_t)
  -> (#type size_t)
  -> Word
  -> IO (#type ssize_t)

sPLICE_F_MOVE :: Word
sPLICE_F_MOVE = (#const "SPLICE_F_MOVE")

sPLICE_F_MORE :: Word
sPLICE_F_MORE = (#const "SPLICE_F_MORE")
#endif

注意: 上面的代码现在 可以正常使用! 感谢Nemo,下方不再有效!

splice按照上面的定义使用两个开放和连接的套接字进行调用(已经使用套接字API
sendrecv调用将它们用于传输最小量的握手数据,或者已转换为句柄并与hGetLine和一起使用hPut),并且不断得到:

Network.Socket.Splice.splice: resource exhausted (Resource temporarily unavailable)

在第一个c_splice呼叫站点:c_splice返回-1并将其设置errno为一个值(可能是EAGAIN),该值resource exhausted | resource temporarily unavailable在查找时会读取。

我测试了splice使用不同Length值的呼叫:10248192


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2020-06-03

共1个答案

小编典典

我不知道Haskell,但是“资源暂时不可用”是EAGAIN

看起来Haskell默认将其套接字设置为非阻塞模式。因此,如果您在没有数据的情况下尝试从其中一个读取数据,或者在缓冲区中的数据已满时尝试向其中之一进行写入操作,则会失败EAGAIN

弄清楚如何将套接字更改为阻塞模式,我敢打赌,您将解决您的问题。

[更新]

或者,在尝试读取或写入套接字之前,先调用selectpoll。但是,您仍然需要处理EAGAIN,因为在极少数情况下,Linux
select会指示套接字已准备就绪,而实际上尚未就绪。

2020-06-03