logging into script

[Dennis J.]

On 01/14/2010 02:51 PM, Igor Sysoev wrote:
> On Thu, Jan 14, 2010 at 01:52:02PM +0100, Dennis J. wrote:
>
>> Why is logging into a pipe considered a waste of CPU?
>> The log parser throws away some data, aggregates the rest and then writes
>> it to a remote database. The "tail -f" approach would waste lokal disk i/o
>> by writing data unnecessarily to disk which i would then have to read again
>> with the script.
>> Why is this considered more efficient than handing the data directly over
>> to a script?
>
> It is not considered as more efficient. It may be more efficient because of
> bulk data processing. Note also, that logged data are written to disk, but
> are not read because they are already in OS cache: they are just copied.
>
> Logging to pipe is a CPU waste because it causes a lot of context switches
> and memory copies for every log operation:

Hm, interesting. I didn't know that writing to a pipe actually forces a 
context switch. I was under the impression that the writing process could 
use up it's time slice to write an arbitrary amount of data into the pipe 
and when the OS scheduler switches to the script it would read all the data 
from that pipe.

The "tail -f" approach looks racy to me though. The log would grow fairly 
fast which means it would probably have to be rotated at least once per 
hour or the disk will fill up. I'm not sure how to process this rotation 
with "tail -f" without potentially missing some data.

Regards,
   Dennis