[lustre-devel] Proposal for JobID caching

Dilger, Andreas andreas.dilger at intel.com
Thu Feb 16 14:30:17 PST 2017

On Feb 16, 2017, at 07:36, Ben Evans <bevans at cray.com> wrote:
> On 2/7/17, 6:01 PM, "Dilger, Andreas" <andreas.dilger at intel.com> wrote:
>> On Feb 2, 2017, at 08:20, Ben Evans <bevans at cray.com> wrote:
>>> https://review.whamcloud.com/#/c/25208/ is a working version of what I
>>> had
>>> proposed, including the suggested changes to default to procname_uid.
>>> This is not perfect, but the performance is much improved over the
>>> current
>>> methods, and unlike inode-based caching Metadata performance isn't
>>> negatively affected.  Multiple simultaneous jobs can be run on the same
>>> file, and get appropriate metrics.
>> I reviewed the patch, and one question that I had is whether you've tested
>> if the JobID is correct when read/write RPCs are generated by readahead or
>> ptlrpcd?  That may be more relevant once the async readahead threads are
>> implemented by Dmitry.  With an inode-based JobID cache then the JobID can
>> (usually) be correctly determined even if the RPC is not generated in the
>> context of the user process.
>> I don't think that is necessarily a fault in your patch, but it may be
>> that
>> the JobID determination hasn't kept pace with other changes in the code.
>> It
>> would be great if you would verify (possibly with a test attached to your
>> patch) that JobID is assigned to all the RPCs that need it.
> I've seen some lustre thread names pop into the JobID under the
> procname_uid scheme when doing something like a dd test.  Filtering them
> out would be relatively straightforward, and keeping the old JobID (if
> available) in the lookup table would be the way to get the most reliable
> info.  There shouldn't be a difference with the current behavior in this
> regard.
> My issue with putting the information in the inode stems from 2 cases, the
> first is RobinHood, which stats *everything*.  In the proposed solution,
> one lookup would be done every 30 seconds.  Storing the inode, it would
> happen for every stat, then never used again.
> The other case is less probable, but still out there, in an environment
> with multiple jobs per node, you may be running two different jobs on the
> same input set, which would corrupt the counting.

If there are two jobs using the same input files, I suspect the second one
would get the data from the client cache, and not log anything on the server
at all.  In any case, I don't think that would be any different than the two
jobs are randomly interleaving their access to the same files on the server.

Conversely, having "ptlrpcd/0" appear in the jobstats doesn't really help
anyone figure out which user/job is causing IO traffic on the server.  If
RPCs generated by ptlrpcd, statahead, and other service threads that do work
on behalf of user processes (including readahead in the near future) have the
proper JobID then that would be much more useful.

Some suggestions on how to handle this, off the top of my head:
- blacklist service thread PIDs at startup in the JobID hash and have them
  get the JobID by some other method (e.g. inode, DLM lock/resource, other)
- store the JobID explicitly with the IO request when it is being put into
  a cache/queue and use this when submitting the RPC if present, otherwise get
  it from the hash

The latter may be preferable, since it doesn't need to do anything for sync
RPCs generated in process context, and avoids an extra lookup when processing
the RPC.  You might consider the first method for debugging when/where such
RPCs are generated, and have the backlisted threads dump a stack once if they
are being looked up in the JobID hash.

Cheers, Andreas

> -Ben
>> Cheers, Andreas
>>> On 1/20/17, 5:00 PM, "Ben Evans" <bevans at cray.com> wrote:
>>>> On 1/20/17, 4:50 PM, "Dilger, Andreas" <andreas.dilger at intel.com>
>>>> wrote:
>>>>> On Jan 18, 2017, at 13:39, Oleg Drokin <oleg.drokin at intel.com> wrote:
>>>>>> On Jan 18, 2017, at 3:08 PM, Ben Evans wrote:
>>>>>>> Overview
>>>>>>>          The Lustre filesystem added the ability to track I/O
>>>>>>> performance of a job across a cluster.  The initial algorithm was
>>>>>>> relatively simplistic:  for every I/O, look up the job ID of the
>>>>>>> process and include it in the RPC being sent to the server.  This
>>>>>>> imposed a non-trivial performance impact on client I/O performance.
>>>>>>>          An additional algorithm was introduced to handle the
>>>>>>> single
>>>>>>> job per node case, where instead of looking up the job ID of the
>>>>>>> process, Lustre simply accesses the value of a variable set through
>>>>>>> the
>>>>>>> proc interface.  This improved performance greatly, but only
>>>>>>> functions
>>>>>>> when a single job is being run.
>>>>>>>          A new approach is needed for multiple job per node
>>>>>>> systems.
>>>>>>> Proposed Solution
>>>>>>>          The proposed solution to this is to create a small
>>>>>>> PID->JobID table in kernel memory.  When a process performs an IO, a
>>>>>>> lookup is done in the table for the PID, if a JobID exists for that
>>>>>>> PID, it is used, otherwise it is retrieved via the same methods as
>>>>>>> the
>>>>>>> original Jobstats algorithm.  Once located the JobID is stored in a
>>>>>>> PID/JobID table in memory. The existing cfs_hash_table structure and
>>>>>>> functions will be used to implement the table.
>>>>>>> Rationale
>>>>>>>          This reduces the number of calls into userspace,
>>>>>>> minimizing
>>>>>>> the time taken on each I/O.  It also easily supports multiple job
>>>>>>> per
>>>>>>> node scenarios, and like other proposed solutions has no issue with
>>>>>>> multiple jobs performing I/O on the same file at the same time.
>>>>>>> Requirements
>>>>>>> ·      Performance cannot significantly detract from baseline
>>>>>>> performance without jobstats
>>>>>>> ·      Supports multiple jobs per node
>>>>>>> ·      Coordination with the scheduler is not required, but
>>>>>>> interfaces
>>>>>>> may be provided
>>>>>>> ·      Supports multiple PIDs per job
>>>>>>> New Data Structures
>>>>>>>          pid_to_jobid {
>>>>>>>                      struct hlist_node pj_hash;
>>>>>>>                      u54 pj_pid;
>>>>>>>                      char pj_jobid[LUSTRE_JOBID_SIZE];
>>>>>>> spinlock_t jp_lock;
>>>>>>>                      time_t jp_time;
>>>>>>> }
>>>>>>> Proc Variables
>>>>>>> Writing to /proc/fs/lustre/jobid_name while not in ³nodelocal² mode
>>>>>>> will cause all entries in the cache for that jobID to be removed
>>>>>>> from
>>>>>>> the cache
>>>>>>> Populating the Cache
>>>>>>>          When lustre_get_jobid is called, the process, and in the
>>>>>>> cached mode, first a check will be done in the cache for a valid
>>>>>>> PID to
>>>>>>> JobID mapping.  If none exists, it uses the same mechanisms to get
>>>>>>> the
>>>>>>> JobID and populates the appropriate PID to JobID map.
>>>>>>> If a lookup is performed and the PID to JobID mapping exists, but is
>>>>>>> more than 30 seconds old, the JobID is refreshed.
>>>>>>> Purging the Cache
>>>>>>>          The cache can be purged of a specific job by writing the
>>>>>>> JobID to the jobid_name proc file.  Any items in the cache that are
>>>>>>> more than 300 seconds out of date will also be purged at this time.
>>>>>> I'd much rather prefer you go to the table that's populated outside
>>>>>> of
>>>>>> the kernel
>>>>>> somehow.
>>>>>> Let's be realistic, poking around in userspace process environments
>>>>>> for
>>>>>> random
>>>>>> strings is not such a great idea at all even though it did look like
>>>>>> a
>>>>>> good idea
>>>>>> in the past for simplicity reasons.
>>>>>> Similar to nodelocal, we probably just switch to a method where you
>>>>>> call a
>>>>>> particular lctl command that would mark the whole session as
>>>>>> belonging
>>>>>> to some job. This might take several forms, e.g. nodelocal itself
>>>>>> could
>>>>>> be extended to only apply to a current namespace/container
>>>>>> But if you do really run different jobs in the global namespace, we
>>>>>> probably can
>>>>>> probably just make the lctl to spawn a shell with commands that all
>>>>>> would
>>>>>> be marked as a particular job? Or we can probably trace the parent of
>>>>>> lctl and
>>>>>> mark that so that all its children become somehow marked too.
>>>>> Having lctl spawn a shell or requiring everything to run in a
>>>>> container
>>>>> is impractical for users, and will just make it harder to use JobID,
>>>>> IMHO.  The job scheduler is _already_ storing the JobID in the process
>>>>> environment so that it is available to all of the threads running as
>>>>> part
>>>>> of the job.  The question is how the job prolog script can communicate
>>>>> the JobID directly to Lustre without using a global /proc file?
>>>>> Doing an
>>>>> upcall to userspace per JobID lookup is going to be *worse* for
>>>>> performance than the current searching through the process
>>>>> environment.
>>>>> I'm not against Ben's proposal to implement a cache in the kernel for
>>>>> different processes.  It is unfortunate that we can't have proper
>>>>> thread-local storage for Lustre, so a hash table is probably
>>>>> reasonable
>>>>> for this (there may be thousands of threads involved).  I don't think
>>>>> the
>>>>> cl_env struct would be useful, since it is not tied to a specific
>>>>> thread
>>>>> (AFAIK), but rather assigned as different threads enter/exit kernel
>>>>> context.  Note that we already have similar time-limited caches for
>>>>> the
>>>>> identity upcall and FMD (lustre/ofd/ofd_fmd.c), so it may be useful to
>>>>> see whether the code can be shared.
>>>> I'll take a look at those, but implementing the hash table was a pretty
>>>> simple solution, I need to work out a few kinks with memory leaks
>>>> before
>>>> doing real performance tests on it to make sure it performs similarly
>>>> to
>>>> nodelocal.
>>>>> Another (not very nice) option to avoid looking through the
>>>>> environment
>>>>> variables (which IMHO isn't so bad, even though the upstream folks
>>>>> don't
>>>>> like it) is to associate the JobID set via /proc with a process group
>>>>> internally and look the PGID up in the kernel to find the JobID.  That
>>>>> can be repeated each time a new JobID is set via /proc, since the PGID
>>>>> would stick around for each new job/shell/process created under the
>>>>> PGID.
>>>>> It won't be as robust as looking up the JobID in the environment, but
>>>>> probably good enough for most uses.
>>>>> I would definitely also be in favor of having some way to fall back to
>>>>> procname_uid if the PGID cannot be found, the job environment
>>>>> variable is
>>>>> not available, and there is nothing in nodelocal.
>>>> That's simple enough.
>> Cheers, Andreas
>> --
>> Andreas Dilger
>> Lustre Principal Architect
>> Intel Corporation

Cheers, Andreas
Andreas Dilger
Lustre Principal Architect
Intel Corporation

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