Question about sendfile

Question about sendfile

[Xiuduan Fang]

[-- Attachment #1: Type: text/plain, Size: 1007 bytes --]


Hi,

I am trying to beat the I/O bottleneck so as to speed up bulk data transfers 
in high speed network. It seems that the system call sendfile() can help to 
reduce CPU utilization and speedup data transfers. But I have one question 
about the system call,

First,  Linux sendfile requires that the input file descriptor cannot be a 
network socket. What are the reasons for such a restriction? Sending a 
socket to a file via zero copy is definitely useful.  Actually this is one 
approach I am trying to do to improve performance.  Some discussions on 
Linux zero copy said this is because it is harder. Sending a socket to a 
file via zero copy needs the support of NICs. I cannot understand this 
explanation. It seems that FreeBSD has implemented bidirectional zero 
copy(http://people.freebsd.org/~ken/zero_copy/#Download). So why Linux does 
not support it? What shall I do to release the restriction that Linux 
enforces on sendfile?

Any hints will be highly appreciated. Thanks.

Xiuduan Fang 

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #2: Xiuduan Fang.vcf --]
[-- Type: text/x-vcard; name="Xiuduan Fang.vcf", Size: 2105 bytes --]

BEGIN:VCARD
VERSION:2.1
N:Fang;Xiuduan
FN:Xiuduan Fang
ORG:University of Virginia;Computer Science Dept
TITLE:2nd Year Graduate
TEL;WORK;VOICE:1-434-982-2296
ADR;WORK:;;151 Engineer's Way, P.O. Box 400740;Charlottesville;VA;22904-4743;USA
LABEL;WORK;ENCODING=QUOTED-PRINTABLE:151 Engineer's Way, P.O. Box 400740=0D=0ACharlottesville, VA 22904-4743=0D=
=0AUSA
KEY;X509;ENCODING=BASE64:
    MIIEYzCCA8ygAwIBAgIQJav9Aj366wHb4hpgZ1JRKDANBgkqhkiG9w0BAQQFADCBzDEXMBUG
    A1UEChMOVmVyaVNpZ24sIEluYy4xHzAdBgNVBAsTFlZlcmlTaWduIFRydXN0IE5ldHdvcmsx
    RjBEBgNVBAsTPXd3dy52ZXJpc2lnbi5jb20vcmVwb3NpdG9yeS9SUEEgSW5jb3JwLiBCeSBS
    ZWYuLExJQUIuTFREKGMpOTgxSDBGBgNVBAMTP1ZlcmlTaWduIENsYXNzIDEgQ0EgSW5kaXZp
    ZHVhbCBTdWJzY3JpYmVyLVBlcnNvbmEgTm90IFZhbGlkYXRlZDAeFw0wNDEwMDQwMDAwMDBa
    Fw0wNDEyMDMyMzU5NTlaMIIBBzEXMBUGA1UEChMOVmVyaVNpZ24sIEluYy4xHzAdBgNVBAsT
    FlZlcmlTaWduIFRydXN0IE5ldHdvcmsxRjBEBgNVBAsTPXd3dy52ZXJpc2lnbi5jb20vcmVw
    b3NpdG9yeS9SUEEgSW5jb3JwLiBieSBSZWYuLExJQUIuTFREKGMpOTgxHjAcBgNVBAsTFVBl
    cnNvbmEgTm90IFZhbGlkYXRlZDEnMCUGA1UECxMeRGlnaXRhbCBJRCBDbGFzcyAxIC0gTWlj
    cm9zb2Z0MRUwEwYDVQQDFAxYaXVkdWFuIEZhbmcxIzAhBgkqhkiG9w0BCQEWFHhmNGNAY3Mu
    dmlyZ2luaWEuZWR1MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDRn6bRIKJguTHWwMQB
    aKdf9VOH3758Ba6owaoGy5ME/fds2ZPTWvuW+IyFskupZ0stK7f9OtzKAi+EFkFlD1umHItr
    XM74PapnYI/8TR/svKbZJLodGNAto9sJjvLQkNK6hwvTp5eBwQ1YgC7GmZHmtshPH8N+8Ast
    xOxoflE6dwIDAQABo4IBBjCCAQIwCQYDVR0TBAIwADCBrAYDVR0gBIGkMIGhMIGeBgtghkgB
    hvhFAQcBATCBjjAoBggrBgEFBQcCARYcaHR0cHM6Ly93d3cudmVyaXNpZ24uY29tL0NQUzBi
    BggrBgEFBQcCAjBWMBUWDlZlcmlTaWduLCBJbmMuMAMCAQEaPVZlcmlTaWduJ3MgQ1BTIGlu
    Y29ycC4gYnkgcmVmZXJlbmNlIGxpYWIuIGx0ZC4gKGMpOTcgVmVyaVNpZ24wEQYJYIZIAYb4
    QgEBBAQDAgeAMDMGA1UdHwQsMCowKKAmoCSGImh0dHA6Ly9jcmwudmVyaXNpZ24uY29tL2Ns
    YXNzMS5jcmwwDQYJKoZIhvcNAQEEBQADgYEASTrowJeKxyNUZbF+AwGXfqXBrOyN3b+3aRDN
    CgSQVp0zaLHwLReTa+3mEnwtrMN6QSM02gPbiuzVkdmGyxmlHAmrHQ2l61fyotoMH47RJbe+
    qzClrcMr2Y9AAyTNeVrvfSZRdKMZ9HFduUu1tn5/FTZFCK8Xoaq3BIo81b8nHGs=


EMAIL;PREF;INTERNET:xf4c@cs.virginia.edu
REV:20050208T032639Z
END:VCARD

Re: Question about sendfile

[Gianni Tedesco]

On Mon, 2005-02-07 at 22:26 -0500, Xiuduan Fang wrote:
> Hi,
> 
> I am trying to beat the I/O bottleneck so as to speed up bulk data transfers 
> in high speed network. It seems that the system call sendfile() can help to 
> reduce CPU utilization and speedup data transfers. But I have one question 
> about the system call,
> 
> First,  Linux sendfile requires that the input file descriptor cannot be a 
> network socket. What are the reasons for such a restriction? Sending a 
> socket to a file via zero copy is definitely useful.  Actually this is one 
> approach I am trying to do to improve performance.  Some discussions on 
> Linux zero copy said this is because it is harder. Sending a socket to a 
> file via zero copy needs the support of NICs. I cannot understand this 
> explanation. It seems that FreeBSD has implemented bidirectional zero 
> copy(http://people.freebsd.org/~ken/zero_copy/#Download). So why Linux does 
> not support it? What shall I do to release the restriction that Linux 
> enforces on sendfile?

>From the URL you posted:

"[zero-copy receive] generally requires some sort of intelligence on the
NIC to make sure that the payload starts in its own buffer.  This is
called "header splitting".  Currently the only NICs with support for
header splitting are Alteon Tigon 2 based boards running slightly
modified firmware."

Perhaps that explains it.

Not to mention the other complications that are involved if you scroll
down the page and read the FAQ.

Have you done any profiling work to see where your CPU cycles are being
spent?

-- 
// Gianni Tedesco (gianni at scaramanga dot co dot uk)
lynx --source www.scaramanga.co.uk/scaramanga.asc | gpg --import