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Full Discussion: ps blocking
Top Forums UNIX for Advanced & Expert Users ps blocking Post 302436988 by steadyonabix on Tuesday 13th of July 2010 03:26:02 PM
Old 07-13-2010
ps blocking

Hi Folks


I have been debugging a script that is called every thirty seconds. Basically it is doing a ps, well two actually, one to file (read by the getline below) and the other into a pipe. The one into the pipe is: -

V_SYSVPS=/usr/sysv/bin/ps

Code:
$V_SYSVPS -p$PIDLIST -o$PSARGS 2>/dev/null | \
nawk -v tmpfile=$TMPFILE -v ta=$(vdate -t -d 197001010000.00 + $CMDTIME) \
                                  -v h=$V_HOSTNAME -v tr=$((COUNT * V_AUTO_MON_FREQUENCY)) -v argfield=$V_AUTO_MON_ARGFIELD \
                                  -v s="MON:1,psm,$V_AUTO_MON_START,$CMDTIME" \
                                  'BEGIN      {
                                                  while (getline < tmpfile)
                                                          l[$1] = $0
                                              }
                                              }'

The problem is that the sysv ps intermittently takes longer than 30 seconds to come back, I have echoed the date to file either side of it and it is definitely blocking on the write to the pipe. I have also trussed the nawk and it hangs on the read from the pipe.

I spotted that the getline does not check for EOF or close the file but it still blocks on the write to the pipe after I fixed it.

Trussing the ps itself shows it goes to the /dev directory for some reason which contains 29000 files and does a statx on each. However most of the time it deals with that in under 2 seconds.

Can anyone tell me if there is anything about the sysv ps that can explain this 30 delay? (Sometimes it takes 20 or so seconds but this completes in time)

Apparently this is using both ps commands because they could not get all the info they wanted from just the one version.
 

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GETDELIM(3)						   BSD Library Functions Manual 					       GETDELIM(3)

NAME
getdelim, getline -- read a delimited record from a stream LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <stdio.h> ssize_t getdelim(char ** restrict lineptr, size_t * restrict n, int delimiter, FILE * restrict stream); ssize_t getline(char ** restrict lineptr, size_t * restrict n, FILE * restrict stream); DESCRIPTION
The getdelim() function reads from the stream until it encounters a character matching delimiter, storing the input in *lineptr. The buffer is NUL-terminated and includes the delimiter. The delimiter character must be representable as an unsigned char. If *n is non-zero, then *lineptr must be pre-allocated to at least *n bytes. The buffer should be allocated dynamically; it must be possible to free(3) *lineptr. getdelim() ensures that *lineptr is large enough to hold the input, updating *n to reflect the new size. The getline() function is equivalent to getdelim() with delimiter set to the newline character. RETURN VALUES
The getdelim() and getline() functions return the number of characters read, including the delimiter. If no characters were read and the stream is at end-of-file, the functions return -1. If an error occurs, the functions return -1 and the global variable errno is set to indi- cate the error. The functions do not distinguish between end-of-file and error, and callers must use feof(3) and ferror(3) to determine which occurred. EXAMPLES
The following code fragment reads lines from a file and writes them to standard output. char *line = NULL; size_t linesize = 0; ssize_t linelen; while ((linelen = getline(&line, &linesize, fp)) != -1) fwrite(line, linelen, 1, stdout); if (ferror(fp)) perror("getline"); ERRORS
[EINVAL] lineptr or n is a NULL pointer. [EOVERFLOW] More than SSIZE_MAX characters were read without encountering the delimiter. The getdelim() and getline() functions may also fail and set errno for any of the errors specified in the routines fflush(3), malloc(3), read(2), stat(2), or realloc(3). SEE ALSO
ferror(3), fgets(3), fopen(3) STANDARDS
The getdelim() and getline() functions conform to IEEE Std 1003.1-2008 (``POSIX.1''). BSD
June 30, 2010 BSD
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