linux-kernel - Re: [PATCHv 2] tcp: properly initialize tcp memory limits part 2 (fix nfs regression)

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Date:	Sat, 3 Mar 2012 17:43:22 +0300
From:	Sergei Trofimovich <slyich@...il.com>
To:	Glauber Costa <glommer@...allels.com>
Cc:	Jason Wang <jasowang@...hat.com>, <linux-kernel@...r.kernel.org>,
	<netdev@...r.kernel.org>, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCHv 2] tcp: properly initialize tcp memory limits part 2
 (fix nfs regression)

On Sat, 3 Mar 2012 11:16:41 -0300
Glauber Costa <glommer@...allels.com> wrote:

> On 03/02/2012 02:50 PM, Sergei Trofimovich wrote:
> >>>> The change looks like a typo (division flipped to multiplication):
> >>>>> limit = nr_free_buffer_pages() / 8;
> >>>>> limit = nr_free_buffer_pages()<<   (PAGE_SHIFT - 10);
> >>>
> >>> Hi, thanks for the reporting. It's not a typo. It was previously:
> >>> sysctl_tcp_mem[1]<<  (PAGE_SHIFT -  7). Looks like we need to do the
> >>> limit check before shift the value. Please try the following patch, thanks.
> >>
> >> Still does not help. I test it by checking sha1sum of a large file over NFS
> >> (small files seem to work simetimes):
> >>
> >>      $ strace sha1sum /gentoo/distfiles/gcc-4.6.2.tar.bz2
> >>      ...
> >>      open("/gentoo/distfiles/gcc-4.6.2.tar.bz2", O_RDONLY
> >>      <HUNG>
> >> After a certain timeout dmesg gets odd spam:
> >> [  314.848094] nfs: server vmhost not responding, still trying
> >> [  314.848134] nfs: server vmhost not responding, still trying
> >> [  314.848145] nfs: server vmhost not responding, still trying
> >> [  314.957047] nfs: server vmhost not responding, still trying
> >> [  314.957066] nfs: server vmhost not responding, still trying
> >> [  314.957075] nfs: server vmhost not responding, still trying
> >> [  314.957085] nfs: server vmhost not responding, still trying
> >> [  314.957100] nfs: server vmhost not responding, still trying
> >> [  314.958023] nfs: server vmhost not responding, still trying
> >> [  314.958035] nfs: server vmhost not responding, still trying
> >> [  314.958044] nfs: server vmhost not responding, still trying
> >> [  314.958054] nfs: server vmhost not responding, still trying
> >>
> >> looks like bogus messages. Might be relevant to mishandled timings
> >> somewhere else or a bug in nfs code.
> >
> > And after 120 seconds hung tasks shows it might be an OOM issue
> > Likely caused by patch, as it's a 2GB RAM +4GB swap amd64 box
> > not running anything heavy:
> 
> That is a bit weird.
> 
> First because with Jason's patch, we should end up with the very same 
> calculation, at the same exact order, as it was in older kernels.
> Second, because by shifting << 10, you should be ending up with very 
> small numbers, effectively having tcp_rmem[1] == tcp_rmem[2], and the 
> same for wmem.
> 
> Can you share which numbers you end up with at 
> /proc/sys/net/ipv4/tcp_{r,w}mem ?
> 

Sure:

    $ cat /proc/sys/net/ipv4/tcp_{r,w}mem
    4096    87380   1999072
    4096    16384   1999072

Nothing special with NFS nere, so I guess it uses UDP.
TCP works fine on machine (I do everything via SSH).

-- 

  Sergei

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