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SubjectRe: [PATCH] ipc/shm.c: add RSS and swap size information to /proc/sysvipc/shm
On Thu, 12 Aug 2010 23:33:29 +0200
Helge Deller <deller@gmx.de> wrote:

> On 08/12/2010 10:10 PM, Andrew Morton wrote:
> > On Wed, 11 Aug 2010 22:13:45 +0200
> > Helge Deller<deller@gmx.de> wrote:
> >
> >> The kernel currently provides no functionality to analyze the RSS
> >> and swap space usage of each individual sysvipc shared memory segment.
> >>
> >> This patch add this info for each existing shm segment by extending
> >> the output of /proc/sysvipc/shm by two columns for RSS and swap.
> >>
> >> Since shmctl(SHM_INFO) already provides a similiar calculation (it
> >> currently sums up all RSS/swap info for all segments), I did split
> >> out a static function which is now used by the /proc/sysvipc/shm
> >> output and shmctl(SHM_INFO).
> >>
> >
> > I suppose that could be useful, although it would be most interesting
> > to hear why _you_ consider it useful?
>
> A reasonable question, and I really should have explained when I did
> send this patch.
>
> In my job I do work for SAP in the SAP LinuxLab
> (http://www.sap.com/linux) and take care of the SAP ERP enterprise
> software on Linux.
> SAP products (esp. the SAP Netweaver ABAP Kernel) uses lots of big
> shared memory segments (we often have Linux systems with >= 16GB shm
> usage). Sometimes we get customer reports about "slow" system responses
> and while looking into their configurations we often find massive
> swapping activity on the system. With this patch it's now easy to see
> from the command line if and which shm segments gets swapped out (and
> how much) and can more easily give recommendations for system tuning.
> Without the patch it's currently not possible to do such shm analysis at
> all.

OK, thanks. copied-n-pasted into changelog ;)

> So, my patch actually does fix a real-world problem.
>
> By the way - I found another bug/issue in /proc/<pid>/smaps as well. The
> kernel currently does not adds swapped-out shm pages to the swap size
> value correctly. The swap size value always stays zero for shm pages.
> I'm currently preparing a small patch to fix that, which I will send to
> linux-mm for review soon.
>
> > But is it useful enough to risk breaking existing code which parses
> > that file? The risk is not great, but it's there.
>
> Sure. The only positive argument is maybe, that I added the new info to
> the end of the lines. IMHO existing applications which parse /proc files
> should always take into account, that more text could follow with newer
> Linux kernels...?

Yeah, they'd be pretty dumb if they failed because new columns appear
in later kernels.

But there's some pretty dumb code out there.

> > This adds 11 new spaces between "perms" and "size", only on 64-bit
> > machines. That was unchangelogged and adds another (smaller) risk of
> > breaking things. Please explain.
>
> Yes, I did added some spaces in front of the "size" field for 64bit
> kernels to get the columns correct if you cat the contents of the file.
> In sysvipc_shm_proc_show() the kernel prints the size value in
> "SPEC_SIZE" format, which is defined like this:
>
> #if BITS_PER_LONG <= 32
> #define SIZE_SPEC "%10lu"
> #else
> #define SIZE_SPEC "%21lu"
> #endif
>
> So, if the header is not adjusted, the columns are not correctly
> aligned. I actually tested this on 32- and 64-bit and it seems correct now.

<copy, paste>




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