Re: netsnmp_large_fd_set_resize() sucking cycles

On 18/10/2014 2:44 a.m., Bill Fenner wrote:
> On Thu, Oct 16, 2014 at 9:48 PM, Jono Poff 
> <[email protected] <mailto:[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?

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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