Re: FSI TX200 S2 and main memory > 4 GB

On Wed, 2007-06-20 at 12:27 -0700, Alexei_Roudnev wrote:
> I have a strong impression, that benefits from HugeTLB-s on a small (<= 4GB)
> and medium size (<= 16 GB) servers are strongly overestimated, while
> problems with them (no automated memory allocation, implementation
> instability and so on) are underestimated.
> 
> Can't find exact numbers, but few charts which I find show 1 - 3 % benefits
> from using hugetlb in few particular cases.
> My own experiments (with Oracle cluster and 4 GB servers) was not clean but
> show, that it's pretty simple to broke OS or slow it down by inproper
> hugetlb configuration.
> 
> So I'd better be very careful with them, and they definitely are not
> mandatory required on 64 bit Linux.

I didn't say they were mandatory requirement, and you are correct that
improper hugetlb configuration can actually slow down a system, but they
are not that difficult to deal with properly if you take the time to
understand the requirements.

I've been running a production Apps 11i environment with 500
manufacturing users for years (since 2001), initially on 32-bit 8GB
system and now on 64-bit 16GB systems, and the difference with hugetbl
is a measurable 15-20%.  Actually, just two weeks ago we upgraded to 10G
and the DBA failed to properly size the new SGA to fit within the
allocated hugepages.  Even though we freed the hugepages to give the
system the memory back, performance was demonstrably slower until we
found time to reboot and properly allocate the memory via hugetbl.

I can send you the sar output from last week showing the 10-25% system
time being used because of the extra overhead of tracking several millon
vma allocs rather than a few thousand.

I'll admit, if your system has tons of free time on it's hands, it's
probably not that noticable, but when it is under stress from tens of
thousands of open cursors from hundreds of users, it can have quite an
impact.

Later,
Tom


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