Opened 2 years ago

Last modified 8 months ago

#34211 new Cleanup/optimization

Performance Regression After Upgrade to Django 3.2 — at Version 1

Reported by: polarmt Owned by: nobody
Component: Database layer (models, ORM) Version: 3.2
Severity: Normal Keywords: performance, regression
Cc: Triage Stage: Accepted
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description (last modified by polarmt)

Overview

We recently upgraded from Django 2.2 to Django 3.2. After the upgrade, several of our APIs slowed down significantly. For instance, our of our APIs increased by 50% from ~0.28 seconds to ~0.41 seconds.

Diagnosis

To diagnose the issue, I created Flamegraphs on the API with Django 2.2 and Django 3.2. The results were nearly identical except for one part:

Code highlighting: 

class ForeignKeyDeferredAttribute(DeferredAttribute):
  def __set__(self, instance, value):
      if instance.__dict__.get(self.field.attname) != value and self.field.is_cached(instance):
          self.field.delete_cached_value(instance)
      instance.__dict__[self.field.attname] = value

​django/db/models/fields/related_descriptors.py in Django 3.2

This code might seem harmless, but it did not scale well when considering how it is called. From my understanding, this scales in relation to the # of rows returned by the query times the # of foreign key fields in the table.

The function below creates an object for every row:

Code highlighting: 

class ModelIterable(BaseIterable):                            
  """Iterable that yields a model instance for each row."""
  def __iter__(self):
      queryset = self.queryset
      db = queryset.db
      compiler = queryset.query.get_compiler(using=db)
      # Execute the query. This will also fill compiler.select, klass_info,
      # and annotations.
      ...
      for row in compiler.results_iter(results):
          obj = model_cls.from_db(db, init_list, row[model_fields_start:model_fields_end])
          for rel_populator in related_populators:
              rel_populator.populate(row, obj)
          ...

Every object created will go through the following logic:

Code highlighting: 

class Model(metaclass=ModelBase):

  def __init__(self, *args, **kwargs):
      # Alias some things as locals to avoid repeat global lookups
      cls = self.__class__
      opts = self._meta
      _setattr = setattr
      _DEFERRED = DEFERRED
      ...

      if not kwargs:
          fields_iter = iter(opts.concrete_fields)
          # The ordering of the zip calls matter - zip throws StopIteration
          # when an iter throws it. So if the first iter throws it, the second
          # is *not* consumed. We rely on this, so don't change the order
          # without changing the logic.
          for val, field in zip(args, fields_iter):
              if val is _DEFERRED:
                  continue
              _setattr(self, field.attname, val)
      ...

If the field is a foreign key, then it will call the overrided __set__. For one API call, we had a filter query on a table with three foreign keys that returns 11k rows. This small change did not scale well with a query.

Verification

We ran the same API call by disabling this overriding:

Code highlighting: 

class ForeignKeyDeferredAttribute(DeferredAttribute):
  def set_fake(self, instance, value):
      if instance.__dict__.get(self.field.attname) != value and self.field.is_cached(instance):
          self.field.delete_cached_value(instance)
      instance.__dict__[self.field.attname] = value

The latency of that API improved from ~0.41 seconds to ~0.31 seconds.

Thoughts

This change does not seem scalable. Even with a moderately sized query and three foreign-key relationships, the query increased the API latency by 50%. We understand the motivation behind the change: Ticket #28147. We were wondering if there is a more performant fix to that problem.

Change History (1)

comment:1 by polarmt, 2 years ago

Description: modified (diff)
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