svn commit: r217830 - head/share/man/man9

mdf at FreeBSD.org mdf at FreeBSD.org
Wed Jan 26 21:14:17 UTC 2011


On Wed, Jan 26, 2011 at 1:10 PM, Robert N. M. Watson
<rwatson at freebsd.org> wrote:
>
> On 26 Jan 2011, at 18:29, mdf at FreeBSD.org wrote:
>
>>> I suppose an important question is now often we see this actually failing
>>
>> I don't believe we've ever seen a memory failure relating to sysctls
>> at Isilon and we've been using the equivalent of this code for a few
>> years.  Our machines aren't low memory but they are under memory
>> pressure sometimes.
>
> The kinds of cases I worry about are things like the tcp connection monitoring sysctls. Most systems have a dozen, hundred, or a thousand connections. Some have half a million or a million. If we switched to requiring wiring every page needed to store that list, it would do terrible things to the system. So really what I have in mind is: either we handle cases like that well, or we put in a clear warning and have obvious failure modes to catch the cases where it didn't work out. In practice, I think we would not want to switch the tcpcb/inpcb sysctl for this reason, but as people say "ah, this is convenient" we need to make sure it's handled well, and easy to debug problems when they do arise.
>

But I think that problem exists today using sysctl for output, since
it's non-iterative.  In fact, it's often worse today, because in
addition to the user-space buffer that needs to be large enough to
hold the output, the kernel needs to malloc(9) a buffer to hold it
before doing the one SYSCTL_OUT at the end that most routines I've
seen use.

For situations like this where there is a lot of output but it doesn't
need to be serialized by a lock held across the whole data fetch, then
yes, using sbuf_new_for_sysctl() would wire more memory.

Thanks,
matthew


More information about the svn-src-head mailing list