I am working on high load daemon development, which listens on UDP and processes packets. Last few months I noticed some strange issue when it takes 500-700 ms to answer packet, while usually it takes 20 ms. I’ve run strace on all daemon processes and found this thing:

13:35:36.979887 stat(“/etc/localtime”, {st_mode=S_IFREG|0644, st_size#09,
…}) = 0
13:35:36.979944 write(3, “[2014-04-15 13:35:36,979] WARNING WORKER 26 –
[pkt#105132/AUTH] loadPresets – memory used: 0 kb\n”, 99) = 99
13:35:37.599793 sendto(10, “Q\0\0\0\rSELECT 1\0”, 14, MSG_NOSIGNAL, NULL,
0) = 14
13:35:37.599865 poll([{fd, events=POLLIN|POLLERR}], 1, -1) = 1 ([{fd, revents=POLLIN}])

You can see that between write and sendto passed around 600 ms. At this time server was not overloaded (LA = 0.4, 16 cores). There were free memory. There were no load on disks. So I took straces of other daemon processes plus database processes. And then did:

grep ’13:35:37.4′ *
grep ’13:35:37.3′ *
grep ’13:35:37.2′ *

None of commans show any lines. So I guess the system was just doing nothing during this 600 ms.

Is there any way to diagnose this issue? What it might be?