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From: Helge Deller on 12 Aug 2010 17:40
On 08/12/2010 10:10 PM, Andrew Morton wrote:
> On Wed, 11 Aug 2010 22:13:45 +0200
> Helge Deller<deller(a)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.

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...?

>> ---
>>
>> shm.c | 63 ++++++++++++++++++++++++++++++++++++++++++---------------------
>> 1 file changed, 42 insertions(+), 21 deletions(-)
>>
>>
>> diff --git a/ipc/shm.c b/ipc/shm.c
>> --- a/ipc/shm.c
>> +++ b/ipc/shm.c
>> @@ -108,7 +108,11 @@ void __init shm_init (void)
>> {
>> shm_init_ns(&init_ipc_ns);
>> ipc_init_proc_interface("sysvipc/shm",
>> - " key shmid perms size cpid lpid nattch uid gid cuid cgid atime dtime ctime\n",
>> +#if BITS_PER_LONG<= 32
>> + " key shmid perms size cpid lpid nattch uid gid cuid cgid atime dtime ctime RSS swap\n",
>> +#else
>> + " key shmid perms size cpid lpid nattch uid gid cuid cgid atime dtime ctime RSS swap\n",
>
> 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.

> This interface is really old and crufty and horrid, but I guess that
> there's not a lot we can do about that :(

Helge
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