Re: [PACTH v2 0/3] Implement /proc/<pid>/totmaps

[Minchan Kim]

On Fri, Aug 19, 2016 at 10:05:32AM +0200, Michal Hocko wrote:
> On Fri 19-08-16 11:26:34, Minchan Kim wrote:
> > Hi Michal,
> > 
> > On Thu, Aug 18, 2016 at 08:01:04PM +0200, Michal Hocko wrote:
> > > On Thu 18-08-16 10:47:57, Sonny Rao wrote:
> > > > On Thu, Aug 18, 2016 at 12:44 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > > > > On Wed 17-08-16 11:57:56, Sonny Rao wrote:
> > > [...]
> > > > >> 2) User space OOM handling -- we'd rather do a more graceful shutdown
> > > > >> than let the kernel's OOM killer activate and need to gather this
> > > > >> information and we'd like to be able to get this information to make
> > > > >> the decision much faster than 400ms
> > > > >
> > > > > Global OOM handling in userspace is really dubious if you ask me. I
> > > > > understand you want something better than SIGKILL and in fact this is
> > > > > already possible with memory cgroup controller (btw. memcg will give
> > > > > you a cheap access to rss, amount of shared, swapped out memory as
> > > > > well). Anyway if you are getting close to the OOM your system will most
> > > > > probably be really busy and chances are that also reading your new file
> > > > > will take much more time. I am also not quite sure how is pss useful for
> > > > > oom decisions.
> > > > 
> > > > I mentioned it before, but based on experience RSS just isn't good
> > > > enough -- there's too much sharing going on in our use case to make
> > > > the correct decision based on RSS.  If RSS were good enough, simply
> > > > put, this patch wouldn't exist.
> > > 
> > > But that doesn't answer my question, I am afraid. So how exactly do you
> > > use pss for oom decisions?
> > 
> > My case is not for OOM decision but I agree it would be great if we can get
> > *fast* smap summary information.
> > 
> > PSS is really great tool to figure out how processes consume memory
> > more exactly rather than RSS. We have been used it for monitoring
> > of memory for per-process. Although it is not used for OOM decision,
> > it would be great if it is speed up because we don't want to spend
> > many CPU time for just monitoring.
> > 
> > For our usecase, we don't need AnonHugePages, ShmemPmdMapped, Shared_Hugetlb,
> > Private_Hugetlb, KernelPageSize, MMUPageSize because we never enable THP and
> > hugetlb. Additionally, Locked can be known via vma flags so we don't need it,
> > either. Even, we don't need address range for just monitoring when we don't
> > investigate in detail.
> > 
> > Although they are not severe overhead, why does it emit the useless
> > information? Even bloat day by day. :( With that, userspace tools should
> > spend more time to parse which is pointless.
> 
> So far it doesn't really seem that the parsing is the biggest problem.
> The major cycles killer is the output formatting and that doesn't sound

I cannot understand how kernel space is more expensive.
Hmm. I tested your test program on my machine.


#!/bin/sh
./smap_test &
pid=$!

for i in $(seq 25)
do
        cat /proc/$pid/smaps > /dev/null
done
kill $pid

root@bbox:/home/barrios/test/smap# time ./s_v.sh
pid:21925
real    0m3.365s
user    0m0.031s
sys     0m3.046s


vs.

#!/bin/sh
./smap_test &
pid=$!

for i in $(seq 25)
do
        awk '/^Rss/{rss+=$2} /^Pss/{pss+=$2} END {}' \
         /proc/$pid/smaps
done
kill $pid

root@bbox:/home/barrios/test/smap# time ./s.sh 
pid:21973

real    0m17.812s
user    0m12.612s
sys     0m5.187s

perf report says

    39.56%  awk        gawk               [.] dfaexec                             
     7.61%  awk        [kernel.kallsyms]  [k] format_decode                       
     6.37%  awk        gawk               [.] avoid_dfa                           
     5.85%  awk        gawk               [.] interpret                           
     5.69%  awk        [kernel.kallsyms]  [k] __memcpy                            
     4.37%  awk        [kernel.kallsyms]  [k] vsnprintf                           
     2.69%  awk        [kernel.kallsyms]  [k] number.isra.13                      
     2.10%  awk        gawk               [.] research                            
     1.91%  awk        gawk               [.] 0x00000000000351d0                  
     1.49%  awk        gawk               [.] free_wstr                           
     1.27%  awk        gawk               [.] unref                               
     1.19%  awk        gawk               [.] reset_record                        
     0.95%  awk        gawk               [.] set_record                          
     0.95%  awk        gawk               [.] get_field                           
     0.94%  awk        [kernel.kallsyms]  [k] show_smap                           

Parsing is much expensive than kernel.
Could you retest your test program?

> like a problem we are not able to address. And I would even argue that
> we want to address it in a generic way as much as possible.

Sure. What solution do you think as generic way?

> 
> > Having said that, I'm not fan of creating new stat knob for that, either.
> > How about appending summary information in the end of smap?
> > So, monitoring users can just open the file and lseek to the (end - 1) and
> > read the summary only.
> 
> That might confuse existing parsers. Besides that we already have
> /proc/<pid>/statm which gives cumulative numbers already. I am not sure
> how often it is used and whether the pte walk is too expensive for
> existing users but that should be explored and evaluated before a new
> file is created.
> 
> The /proc became a dump of everything people found interesting just
> because we were to easy to allow those additions. Do not repeat those
> mistakes, please!
> -- 
> Michal Hocko
> SUSE Labs