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Re: advise on future of glibc (mktime issue)

On Tue, Jun 07, 2005 at 10:35:48AM -0400, wrote:
>    Hola:
>   This seems to be the right mailing list for this message, please let 
> me know otherwise.
>   We recently ran into some problems, and their root lies in glibc.
>   I will ask my questions first, then I will elaborate on the problem.
>   1.-  What is the relationship between RedHat and glibc (besides 
> hosting the project)?,
>   2.-  do RH patches make it to the general glibc releases ?, does this 
> mean that eventually other Linux distributions will get those changes ? 
> (I think the answer to the second part is almost an obvious YES, but I 
> have to ask)
>   3.- do other developers besides redhat's have the chance to 
> review/approve RH's changes before the make it to the main tree ?
>   We found our problem when migrating from RH7.3 to RHEL4 and FC3. Our 
> application slowed down significantly, a process that takes 1 day in 
> RH7.3 now takes 3-4 days in FC3 and RHEL4.
>    After debugging I see the problem shows up because we call millions 
> of times mktime (no way to avoid that for us). The patched glibc (I 
> think by RH) has mktime checking for changes on the system's timezone, 
> this is done by checking the file: /etc/localtime which is either a link 
> or a copy of the local timezone file from /usr/share/zoneinfo, this is a 
> call to the file system via stat64(), hence our program is accessing 
> millions of times the file system and crawling in pain. The old glibc, 
> used in RH7.3 and used in Suse Enterprise 9 does not check for changes 
> to this file with every call to mktime.
>  I was able to eliminate the checks by temporarily setting TZ to GMT, 
> but as I understand that is no longer suggested/supported, and RH7.3 was 
> still faster.
>  I contacted RH and they say they will not change the behavior of their 
> glibc because they prefer "correctness over performance" and they 
> believe glibc should be checking for changes to the timezone. Their 
> suggestions are: build our own glibc and loose RH support, rewrite 
> mktime for our own use (I tried that but seemed a little complicated and 
> looses the benefit of the library per se), and lastly they suggested to 
> set TZ=/etc/localtime (which makes the program run a little faster, but 
> not as fast as RH7.3).
>  It is widely known that FC1,2,3 and RHEL* are slow systems, and I 
> always figured it was for the better, then I question, how may of this 
> little changes in glibc exists in RH's patches that by themselves may 
> not be a big performance hit, but put them together and you get a slower 
> system ?
>   So our solution could be to migrate to another Linux platform, but if 
> the changes that RH made are going to show up in the library anyway, we 
> would be just delaying the problem.
>  My little and simple program that shows the performance hit (I'm 
> leaving all the includes that my main program uses):

Are you 100% sure that mktime is the only cause of slowdown in your
application on RHEL 4? If it is the case, it shouldn't be too hard
to work around it.


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