Re: [Xen-devel] [PATCH v3] tools/xenconsoled: Increase file descriptor limit

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 19/02/15 17:56, Andrew Cooper wrote:
> On 19/02/15 16:30, Ian Jackson wrote:
>> Wei Liu writes ("Re: [PATCH v3] tools/xenconsoled: Increase file descriptor 
>> limit"):
>>> On Tue, Feb 17, 2015 at 05:55:52PM +0000, Andrew Cooper wrote:
>>>> XenServer's VM density testing uncovered a regression when moving from
>>>> sysvinit to systemd where the file descriptor limit dropped from 4096 to
>>>> 1024. (XenServer had previously inserted a ulimit statement into its
>>>> initscripts.)
>> I think that putting something like this in the initscripts is a good
>> idea.
>>
>> I don't like the idea of this kind of resource limit mangling (quite
>> vigorous, too!) being buried in the C code.
>>
>>>> One solution is to use LimitNOFILE=4096 in xenconsoled.service to match the
>>>> lost ulimit, but that is only a stopgap solution.
>> Why is this only a stopgap solution ?
> It is yet another place with an arbitrary limit, which is one more
> moving part to go wrong.
>
>>>> As Xenconsoled genuinely needs a large number of file descriptors
>>>> if a large number of domains are running, and is well behaved with
>>>> its descriptors, attempt to up the limit to the system maximum.
>> Perhaps you mean it should be "unlimited" but that doesn't seem right
>> either.
> Where do I perhaps mean "unlimited"?
>
> Attempting to set NOFILE with RLIM_INFINITY is an unconditional failure,
> and there is unfortunately no RLIM_SYSTEM_MAX.
>
>> The value should be some multiple of the maximum number of
>> domains.
> The maximum number of domains (limited by the domid abi) is 32751
>
> However, a brief grep through the code shows that the fd situation is
> far worse than I first thought.  There are 4 FDs held open per
> domain[1], and some overhead for hypervisor logging, gntdev and privcmd.
>
> That would put the number of fds needed at 131004 + overhead (20
> perhaps?).  This is substantially larger than the default on Linux, but
> within the default system max of 2^20.
>
> ~Andrew

Ping?

>
> [1] along with a todo note suggesting that opening evtchn for each
> domain is inefficient.
>
>
>
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