On Tue, Mar 1, 2011 at 9:23 PM, wren ng thornton
On 3/1/11 1:35 AM, Bryan O'Sullivan wrote:
I want to see four entry points for writing:
fdWrite :: Strict.ByteString -> IO Int fdWriteAll :: Strict.ByteString -> IO () fdWritev :: [Strict.ByteString] -> IO Int -- turn the list into an iovec, then call writev fdWritevAll :: [Strict.ByteString] -> IO ()
Using writev requires the length of the list in order to get a count of chunks, which forces us to hold the whole list/lazy-bytestring in memory at once and also adds O(n) time for traversing it. Also it'd require converting each of the ByteString structs into iovec structs (whereas using write allows this to be unpacked into the call frames for write).
What's the benefit of doing this? Is writev that much more efficient than Haskell code with the same semantics[1]?
The benefit of using 'writev' over multiple calls to 'write' is the 'writev' is frequently a single kernel call - avoiding multiple context switches. Whether or not it is worth it to hang on to the entire bytestring to get this advantage probably depends on the circumstance. Antoine