Re: netsnmp_large_fd_set_resize() sucking cycles

On Mon, Oct 20, 2014 at 9:23 PM, Jono Poff <[email protected]>
wrote:

>  On 18/10/2014 2:44 a.m., Bill Fenner wrote:
>
>  On Thu, Oct 16, 2014 at 9:48 PM, Jono Poff <[email protected]>
> wrote:
>
>>
>> Hi,
>>
>> I'm investigating excessive cpu usage in some server software on Solaris
>> 10.
>>
>> By prstat, the server process is using 13% (out of 25%, since it is a
>> single thread on 4 cores).
>>
>> By Brendan Gregg's hotuser script (see
>> http://www.brendangregg.com/DTrace/hotuser) the function
>> netsnmp_large_fd_set_resize() is using up to 80% of that.
>>
>> By the same script, libnetsnmp.so is using over 80%.
>>
>> The context of the call is that I'm using snmp_select_info() to
>> integrate SNMP into my own event loop which uses select().  (This is
>> pretty much as described under "Own custom event loop" at
>> http://vincent.bernat.im/en/blog/2012-snmp-event-loop.html)
>>
>> netsnmp_large_fd_set_resize() is called from snmp_read() and run_alarms().
>>
>> The app is using net-snmp-5.6.1.1.  Would upgrading the library (not a
>> trivial task) improve the situation?
>>
>> Can anyone suggest what can be done to alleviate this.  64 bit Solaris
>> has a fixed FD_SETSIZE of 64k, so what does
>> netsnmp_large_fd_set_resize() need to resize that for?  Why does it
>> require space for more than 65535 handles?
>>
>
>  It's spending time resizing from 0 to FD_SETSIZE.  The change at
> http://sourceforge.net/p/net-snmp/code/ci/4224cabcb112d0d437f9ca9f4d3430501d1c8750/
> should help you.
>
>    Bill
>
>
>
> Awesome thankyou!  from 80% of total cpu time in this function down to
> 0.8% :D
>
> Just a couple more questions (and I haven't fully tested snmp
> functionality with the change so there may be more, but) ...
>
> Why doesn't the latest version of the library have this patch?
>

It is in 5.7.3.rc1.  Earlier versions don't have it because time travel
doesn't work yet :-)

  Bill


> Why does snmplib recreate the fdset each time?  In a single threaded app
> at least, couldn't this be a static object?
>
> Cheers and thanks again,
> Jono
>
>
>
>
>
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