Re: [PATCH 0/3 v3] dcache: make it more scalable on large system

[Waiman Long]

On 05/29/2013 12:13 PM, Andi Kleen wrote:
> > The d_path() is called by perf_event_mmap_event() which translates
> > VMA to its file path for memory segments backed by files. As perf is
> > not just for sampling data within the kernel, it can also be used
> > for checking access pattern in the user space. As a result, it needs
> > to map VMAs back to the backing files to access their symbols
> > information. If d_path() is not the right function to call for this
> > purpose, what other alternatives do we have?
> In principle it should be only called for new file mappings
> getting maped.  Do you really have that many new file mappings all
> the time? Or is this related to program startup?

The AIM7 benchmark that I used runs a large number of relatively short 
jobs. I think each time a new job is spawned, the file mappngs have to 
be redone again. It is probably not a big problem for long running 
processes.

> > My patch set consists of 2 different changes. The first one is to
> > avoid taking the d_lock lock when updating the reference count in
> > the dentries. This particular change also benefit some other
> > workloads that are filesystem intensive. One particular example is
> > the short workload in the AIM7 benchmark. One of the job type in the
> > short workload is "misc_rtns_1" which calls security functions like
> > getpwnam(), getpwuid(), getgrgid() a couple of times. These
> > functions open the /etc/passwd or /etc/group files, read their
> > content and close the files. It is the intensive open/read/close
> > sequence from multiple threads that is causing 80%+ contention in
> > the d_lock on a system with large number of cores. The MIT's
> > MOSBench paper also outlined dentry reference counting as a
> The paper was before Nick Piggin's RCU (and our) work on this.
> Modern kernels do not have dcache problems with mosbench, unless
> you run weird security modules like SMACK that effectively
> disable dcache RCU.

I had tried, but not yet able to run the MOSBench myself. Thank for 
letting me know that the dcache problem wrt MOSBench was fixed.

> BTW lock elision may fix these problems anyways, in a much
> simpler way.

I will certainly hope so. However, there will still be a lot of 
computers out there running pre-Haswell Intel chips. For them, locking 
is still a problem that need to be solved.

Regards,
Longman
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