Causing a process switch to test a theory.

[Matthew Hagerty]

Steve Watt wrote:

>In <423DE326.9000203 at digitalstratum.com>,
>  matthew at digitalstratum.com wrote:
>  
>
>>Knowing if Apache could possibly write interleaved logs when writing to 
>>a pipe is critical to a program I'm developing which receives log 
>>entries from Apache via a pipe.
>>    
>>
>
>That's another layer of indirection, though.  If all of the children
>have separate pipes to the parent, and then the parent logs to your
>program, all should be fine.
>
>But at the kernel level, yes, writes longer than PIPE_BUF might get
>interleaved.  The longer the write, the higher the probability, so
>for your test, if you can generate, say, 10K writes over and over,
>you can probably trip it.
>
>  
>
That's an interesting idea about each child having a pipe open back to 
the parent, but I don't think that's what is happening.  I'm pretty sure 
each child has the logging pipe open directly.  Here is an fstat:

There is a grouping like this for each Apache child process that's runing:
genie    httpd      61320 root /             2 drwxr-xr-x     512  r
genie    httpd      61320   wd /             2 drwxr-xr-x     512  r
genie    httpd      61320 text /usr     5110792 -rwxr-xr-x  9766440  r
genie    httpd      61320    0 /dev          8 crw-rw-rw-    null  r
genie    httpd      61320    1 /dev          8 crw-rw-rw-    null  w
genie    httpd      61320    2 /usr     5110968 -rw-r--r--  494192  w
genie    httpd      61320   15 /usr     5110968 -rw-r--r--  494192  w
genie    httpd      61320   16 /usr     5110969 -rw-r--r--  451616  w
genie    httpd      61320   17 /usr     5110981 -rw-r--r--  1056219  w
genie    httpd      61320   18 /usr     5111084 -rw-r--r--  3886425  w
genie    httpd      61320   19* internet stream tcp c5d1e2d0
genie    httpd      61320   20 /usr     5111178 prw-rw-rw-       0  w
genie    httpd      61320   21 /usr     5111178 prw-rw-rw-       0  w
genie    httpd      61320   22 /usr     5111178 prw-rw-rw-       0  w
genie    httpd      61320   23 /usr     5111178 prw-rw-rw-       0  w
genie    httpd      61320   24 /usr     5110835 -rw-r--r--  573236  w

My pipe listener (fd 0), which each child has open 4 times on fd 20 - 23:
root     pglogd       519 root /             2 drwxr-xr-x     512  r
root     pglogd       519   wd /             2 drwxr-xr-x     512  r
root     pglogd       519 text /usr     8528219 -rwxr-xr-x   22793  r
root     pglogd       519    0 /usr     5111178 prw-rw-rw-       0 rw
root     pglogd       519    1 /usr     5111198 -rw-r--r--       0  r
root     pglogd       519    2 /usr     5111198 -rw-r--r--       0  w
root     pglogd       519    3* local stream c5dcb7a8 <-> c5dcb71c

Matthew