Luis Claudio R. Goncalves wrote:
John Sigler wrote:My platform: Linux 2.6.22.1-rt9 + glibc 2.3.6 I'm writing a "real-time" application that runs with high priority (40 or 80 in SCHED_FIFO). I use syslog(3) to log. As far as I can see, syslog(3) blocks when the local socket becomes full (11 messages on my system). Consider the following program. #include <syslog.h> int main(void) { int i; for (i=0; i < 500; ++i) syslog(LOG_INFO, "I=%d", i); return 0; }John, I use glibc-2.6-4 and your test program worked fine. Probably
When you write "your test program worked fine" do you mean that the process ran and exited with 0?
your man page (3) for syslog may have details on this feature.
What feature are you referring to?
You probably have reached one of the old approaches to avoid DoS via syslog. Limited message rate would be a reasonable assumption.
Did you kill syslogd before running the test program?
One way to better understand what's going on is to either setup syslogd or your client program (via openlog) to enter in debug mode an print to stderr.
In my example, syslogd is not running. See below.
I kill syslogd, then start the above program. It blocks. I kill it, then start syslogd, which grabs the following messages. Nov 13 11:18:57 venus a.out: I=0 Nov 13 11:18:57 venus a.out: I=1 Nov 13 11:18:57 venus a.out: I=2 Nov 13 11:18:57 venus a.out: I=3 Nov 13 11:18:57 venus a.out: I=4 Nov 13 11:18:57 venus a.out: I=5 Nov 13 11:18:57 venus a.out: I=6 Nov 13 11:18:57 venus a.out: I=7 Nov 13 11:18:57 venus a.out: I=8 Nov 13 11:18:57 venus a.out: I=9 Nov 13 11:18:57 venus a.out: I=10 (The process managed to write 11 messages before being blocked.) I expected a local socket to buffer way more than 11 messages. I expected a local socket to discard new messages when it is full. Apparently, these expectations are incorrect. I can see how this behavior can become a problem: Consider process A with prio 80 in SCHED_FIFO and process B with prio 10 in SCHED_FIFO, i.e. process B only runs when A does not want the CPU. (syslogd is in SCHED_OTHER.) 'A' runs, starts logging, and reaches the 11-message limit. The call to write() blocks, and 'A' is put to sleep. The scheduler then picks 'B' because it has higher priority than syslogd. If B runs "forever", 'A' will never get the CPU back. Is this scenario possible? Is this what is called priority inversion?
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