netdev - Re: Splice status

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Date:	Mon, 26 Jul 2010 22:50:01 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	Ofer Heifetz <oferh@...vell.com>, Changli Gao <xiaosuo@...il.com>,
	Jens Axboe <axboe@...nel.dk>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Splice status

Le lundi 26 juillet 2010 à 22:37 +0200, Jarek Poplawski a écrit :
> Ofer Heifetz wrote, On 25.07.2010 16:47:
> 
> > Hi Eric,
> > 
> > Still trying to get better performance with splice, I noticed that when using splice there are twice the file size memcpy (placed a counter in memcpy), I verified it via samba file transfer and splice-fromnet/out.
> > 
> > I also used the splice-fromnet/out and using ftrace I did notice that data is copied twice using these routines: skb_splice_bits, pipe_to_file.
> 
> 
> I'm not sure you're using optimal NIC for splice, which should use
> skbs with almost all data paged (non-linear), like niu or myri10ge.
> 
> Jarek P.

Yes, I dont think splice() _should_ be faster, with a NIC delivering
frames of 1460 (or less bytes), when disk IO should be performed with 4
Kbytes blocs (or a multiple) to get good performance.

sendfile(file -> socket) is fast because blocs are pages, but
splice(socket -> file) is not fast, unless the NIC is able to perform
tcp receive offload.

To take an analogy, think about libc stdio versus read(2)/write(2)
syscalls. stdio, while doing copies in intermediate buffers, is able to
be faster than read()/write() in most cases.

Using splice() with 1460 bytes frames is like using read()/write()
instead of nice sized buffers given by stdio layer.

zero-copy can hurt badly if the IO sizes are not page aligned.

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