linux-kernel - Re: sendfile() expert advice sought

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 17 Feb 2010 18:57:10 -0600
From:	Robert Hancock <hancockrwd@...il.com>
To:	"Patrick J. LoPresti" <lopresti@...il.com>
CC:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: sendfile() expert advice sought

On 02/16/2010 01:53 PM, Patrick J. LoPresti wrote:
> Executive summary:  Can I get the benefits of sendfile() for anonymous pages?
>
> I have an application that generates hundreds of gigabytes of data per
> hour.  I want to push that data out over a TCP socket.  (The network
> connection will be fast; multiple bonded GigE lines or 10GigE.)
>
> I gather that sendfile() is pretty efficient, so I would like to use
> it.  But I do not want to write all of my data to disk first.  So I am
> considering an approach like this:
>
>    int fd = shm_open("/foo", O_RDWR|O_TRUNC);
>    ftruncate(fd, length);
>    void *p = mmap (0, length, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
>    // (fill memory block at p with some data)
>    sendfile(fd, sock, 0, length);
>
> Questions:
>
> 1) Will this work at all?  (Some on-line sources suggest sendfile()
> does not work with tmpfs files.  But I think this was fixed at some
> point...)
>
> 2) Will it provide zero-copy behavior, or does the fact that the pages
> are mapped in my process cause sendfile() to copy them?
>
> 3) If it is zero-copy, what happens if I overwrite the memory block
> after sendfile() returns?  Do I risk corrupting my data?  (In
> particular, suppose I have TCP_CORK set on the socket.  Will
> sendfile() return before all of the data has actually been sent,
> giving me a window to corrupt my data?  If so, how do I know when it
> is "safe" to re-use the memory?)
>
> 4) If sendfile() is not zero-copy in this example, would I expect a
> performance boost anyway, because sendfile() does not need to crawl
> page tables or something?
>
> Any responses or references will be appreciated.

I can't really answer definitively, but as far as I know you wouldn't 
get any magic performance benefits from playing games with sendfile, 
splice, etc. as long as the pages were mapped into userspace already, 
they only really help when the data being written out is coming from 
another file, pipe, etc. not when your application is generating the 
data internally.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/