diff --git a/lib/spack/spack/environment/environment.py b/lib/spack/spack/environment/environment.py
index 5a271fc6dd..5fce76118c 100644
--- a/lib/spack/spack/environment/environment.py
+++ b/lib/spack/spack/environment/environment.py
@@ -9,6 +9,7 @@
 import re
 import shutil
 import sys
+import time
 
 import ruamel.yaml as yaml
 import six
@@ -17,6 +18,8 @@
 import llnl.util.filesystem as fs
 import llnl.util.tty as tty
 
+import spack.bootstrap
+import spack.compilers
 import spack.concretize
 import spack.config
 import spack.error
@@ -28,10 +31,13 @@
 import spack.spec
 import spack.stage
 import spack.store
+import spack.subprocess_context
 import spack.user_environment as uenv
+import spack.util.cpus
 import spack.util.environment
 import spack.util.hash
 import spack.util.lock as lk
+import spack.util.parallel
 import spack.util.path
 import spack.util.spack_json as sjson
 import spack.util.spack_yaml as syaml
@@ -1111,14 +1117,57 @@ def _concretize_separately(self, tests=False):
                 self._add_concrete_spec(s, concrete, new=False)
 
         # Concretize any new user specs that we haven't concretized yet
-        concretized_specs = []
+        arguments, root_specs = [], []
         for uspec, uspec_constraints in zip(
-                self.user_specs, self.user_specs.specs_as_constraints):
+                self.user_specs, self.user_specs.specs_as_constraints
+        ):
             if uspec not in old_concretized_user_specs:
-                concrete = _concretize_from_constraints(uspec_constraints, tests=tests)
-                self._add_concrete_spec(uspec, concrete)
-                concretized_specs.append((uspec, concrete))
-        return concretized_specs
+                root_specs.append(uspec)
+                arguments.append((uspec_constraints, tests))
+
+        # Ensure we don't try to bootstrap clingo in parallel
+        if spack.config.get('config:concretizer') == 'clingo':
+            with spack.bootstrap.ensure_bootstrap_configuration():
+                spack.bootstrap.ensure_clingo_importable_or_raise()
+
+        # Ensure all the indexes have been built or updated, since
+        # otherwise the processes in the pool may timeout on waiting
+        # for a write lock. We do this indirectly by retrieving the
+        # provider index, which should in turn trigger the update of
+        # all the indexes if there's any need for that.
+        _ = spack.repo.path.provider_index
+
+        # Ensure we have compilers in compilers.yaml to avoid that
+        # processes try to write the config file in parallel
+        _ = spack.compilers.get_compiler_config()
+
+        # Solve the environment in parallel on Linux
+        start = time.time()
+        max_processes = min(
+            max(len(arguments), 1),  # Number of specs
+            16  # Cap on 16 cores
+        )
+
+        # TODO: revisit this print as soon as darwin is parallel too
+        msg = 'Starting concretization'
+        if sys.platform != 'darwin':
+            msg = msg + ' pool with {0} processes'.format(
+                spack.util.parallel.num_processes(max_processes=max_processes)
+            )
+        tty.msg(msg)
+
+        concretized_root_specs = spack.util.parallel.parallel_map(
+            _concretize_task, arguments, max_processes=max_processes
+        )
+
+        finish = time.time()
+        tty.msg('Environment concretized in {0} sec.'.format(finish - start))
+        results = []
+        for abstract, concrete in zip(root_specs, concretized_root_specs):
+            self._add_concrete_spec(abstract, concrete)
+            results.append((abstract, concrete))
+
+        return results
 
     def concretize_and_add(self, user_spec, concrete_spec=None, tests=False):
         """Concretize and add a single spec to the environment.
@@ -1962,6 +2011,12 @@ def _concretize_from_constraints(spec_constraints, tests=False):
             invalid_constraints.extend(inv_variant_constraints)
 
 
+def _concretize_task(packed_arguments):
+    spec_constraints, tests = packed_arguments
+    with tty.SuppressOutput(msg_enabled=False):
+        return _concretize_from_constraints(spec_constraints, tests)
+
+
 def make_repo_path(root):
     """Make a RepoPath from the repo subdirectories in an environment."""
     path = spack.repo.RepoPath()
diff --git a/lib/spack/spack/fetch_strategy.py b/lib/spack/spack/fetch_strategy.py
index 6db41e2328..436fbf39bf 100644
--- a/lib/spack/spack/fetch_strategy.py
+++ b/lib/spack/spack/fetch_strategy.py
@@ -48,7 +48,7 @@
 import spack.util.crypto as crypto
 import spack.util.pattern as pattern
 import spack.util.url as url_util
-import spack.util.web as web_util
+import spack.util.web
 import spack.version
 from spack.util.compression import decompressor_for, extension
 from spack.util.executable import CommandNotFoundError, which
@@ -350,8 +350,8 @@ def _existing_url(self, url):
         else:
             # Telling urllib to check if url is accessible
             try:
-                url, headers, response = web_util.read_from_url(url)
-            except web_util.SpackWebError:
+                url, headers, response = spack.util.web.read_from_url(url)
+            except spack.util.web.SpackWebError:
                 msg = "Urllib fetch failed to verify url {0}".format(url)
                 raise FailedDownloadError(url, msg)
             return (response.getcode() is None or response.getcode() == 200)
@@ -380,8 +380,8 @@ def _fetch_urllib(self, url):
 
         # Run urllib but grab the mime type from the http headers
         try:
-            url, headers, response = web_util.read_from_url(url)
-        except web_util.SpackWebError as e:
+            url, headers, response = spack.util.web.read_from_url(url)
+        except spack.util.web.SpackWebError as e:
             # clean up archive on failure.
             if self.archive_file:
                 os.remove(self.archive_file)
@@ -571,7 +571,7 @@ def archive(self, destination):
         if not self.archive_file:
             raise NoArchiveFileError("Cannot call archive() before fetching.")
 
-        web_util.push_to_url(
+        spack.util.web.push_to_url(
             self.archive_file,
             destination,
             keep_original=True)
@@ -1388,12 +1388,12 @@ def fetch(self):
         basename = os.path.basename(parsed_url.path)
 
         with working_dir(self.stage.path):
-            _, headers, stream = web_util.read_from_url(self.url)
+            _, headers, stream = spack.util.web.read_from_url(self.url)
 
             with open(basename, 'wb') as f:
                 shutil.copyfileobj(stream, f)
 
-            content_type = web_util.get_header(headers, 'Content-type')
+            content_type = spack.util.web.get_header(headers, 'Content-type')
 
         if content_type == 'text/html':
             warn_content_type_mismatch(self.archive_file or "the archive")
diff --git a/lib/spack/spack/hash_types.py b/lib/spack/spack/hash_types.py
index 12a57640bd..ab41e1869e 100644
--- a/lib/spack/spack/hash_types.py
+++ b/lib/spack/spack/hash_types.py
@@ -44,6 +44,13 @@ def attr(self):
     deptype=('build', 'link', 'run'), package_hash=False, name='build_hash')
 
 
+#: Hash descriptor used only to transfer a DAG, as is, across processes
+process_hash = SpecHashDescriptor(
+    deptype=('build', 'link', 'run', 'test'),
+    package_hash=False,
+    name='process_hash'
+)
+
 #: Full hash used in build pipelines to determine when to rebuild packages.
 full_hash = SpecHashDescriptor(
     deptype=('build', 'link', 'run'), package_hash=True, name='full_hash')
diff --git a/lib/spack/spack/modules/common.py b/lib/spack/spack/modules/common.py
index 21cb527b65..0e9c8eac68 100644
--- a/lib/spack/spack/modules/common.py
+++ b/lib/spack/spack/modules/common.py
@@ -2,7 +2,6 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
 """Here we consolidate the logic for creating an abstract description
 of the information that module systems need.
 
@@ -43,7 +42,7 @@
 
 import spack.build_environment as build_environment
 import spack.config
-import spack.environment as ev
+import spack.environment
 import spack.error
 import spack.paths
 import spack.projections as proj
@@ -698,12 +697,13 @@ def environment_modifications(self):
         spec = self.spec.copy()  # defensive copy before setting prefix
         if use_view:
             if use_view is True:
-                use_view = ev.default_view_name
+                use_view = spack.environment.default_view_name
 
-            env = ev.active_environment()
+            env = spack.environment.active_environment()
             if not env:
-                raise ev.SpackEnvironmentViewError("Module generation with views "
-                                                   "requires active environment")
+                raise spack.environment.SpackEnvironmentViewError(
+                    "Module generation with views requires active environment"
+                )
 
             view = env.views[use_view]
 
diff --git a/lib/spack/spack/solver/asp.py b/lib/spack/spack/solver/asp.py
index c091d0b89d..8bafa075db 100644
--- a/lib/spack/spack/solver/asp.py
+++ b/lib/spack/spack/solver/asp.py
@@ -334,7 +334,7 @@ def solve(
         self.control.configuration.asp.trans_ext = 'all'
         self.control.configuration.asp.eq = '5'
         self.control.configuration.configuration = 'tweety'
-        self.control.configuration.solve.parallel_mode = '2'
+        self.control.configuration.solve.parallel_mode = '1'
         self.control.configuration.solver.opt_strategy = "usc,one"
 
         # set up the problem -- this generates facts and rules
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 8b13374430..a153ffe50f 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -1567,6 +1567,14 @@ def build_hash(self, length=None):
         """
         return self._cached_hash(ht.build_hash, length)
 
+    def process_hash(self, length=None):
+        """Hash used to store specs in environments.
+
+        This hash includes build and test dependencies and is only used to
+        serialize a spec and pass it around among processes.
+        """
+        return self._cached_hash(ht.process_hash, length)
+
     def full_hash(self, length=None):
         """Hash  to determine when to rebuild packages in the build pipeline.
 
@@ -1832,6 +1840,7 @@ def node_dict_with_hashes(self, hash=ht.dag_hash):
                 not self._hashes_final)                     # lazily compute
             if write_full_hash:
                 node[ht.full_hash.name] = self.full_hash()
+
             write_build_hash = 'build' in hash.deptype and (
                 self._hashes_final and self._build_hash or  # cached and final
                 not self._hashes_final)                     # lazily compute
@@ -1839,8 +1848,12 @@ def node_dict_with_hashes(self, hash=ht.dag_hash):
                 node[ht.build_hash.name] = self.build_hash()
         else:
             node['concrete'] = False
+
         if hash.name == 'build_hash':
             node[hash.name] = self.build_hash()
+        elif hash.name == 'process_hash':
+            node[hash.name] = self.process_hash()
+
         return node
 
     def to_yaml(self, stream=None, hash=ht.dag_hash):
@@ -1974,7 +1987,8 @@ def read_yaml_dep_specs(deps, hash_type=ht.dag_hash.name):
                 # new format: elements of dependency spec are keyed.
                 for key in (ht.full_hash.name,
                             ht.build_hash.name,
-                            ht.dag_hash.name):
+                            ht.dag_hash.name,
+                            ht.process_hash.name):
                     if key in elt:
                         dep_hash, deptypes = elt[key], elt['type']
                         hash_type = key
@@ -4430,7 +4444,7 @@ def __hash__(self):
         return hash(lang.tuplify(self._cmp_iter))
 
     def __reduce__(self):
-        return _spec_from_dict, (self.to_dict(hash=ht.build_hash),)
+        return _spec_from_dict, (self.to_dict(hash=ht.process_hash),)
 
 
 def merge_abstract_anonymous_specs(*abstract_specs):
diff --git a/lib/spack/spack/test/cmd/dev_build.py b/lib/spack/spack/test/cmd/dev_build.py
index f8be2364d9..765d4dc81b 100644
--- a/lib/spack/spack/test/cmd/dev_build.py
+++ b/lib/spack/spack/test/cmd/dev_build.py
@@ -245,7 +245,7 @@ def test_dev_build_env_version_mismatch(tmpdir, mock_packages, install_mockery,
 
         env('create', 'test', './spack.yaml')
         with ev.read('test'):
-            with pytest.raises(spack.spec.UnsatisfiableVersionSpecError):
+            with pytest.raises(RuntimeError):
                 install()
 
 
diff --git a/lib/spack/spack/user_environment.py b/lib/spack/spack/user_environment.py
index 23fb6529d8..55f516b732 100644
--- a/lib/spack/spack/user_environment.py
+++ b/lib/spack/spack/user_environment.py
@@ -5,7 +5,7 @@
 import os
 import sys
 
-import spack.build_environment as build_env
+import spack.build_environment
 import spack.config
 import spack.util.environment as environment
 import spack.util.prefix as prefix
@@ -85,13 +85,13 @@ def environment_modifications_for_spec(spec, view=None):
     # Let the extendee/dependency modify their extensions/dependents
     # before asking for package-specific modifications
     env.extend(
-        build_env.modifications_from_dependencies(
+        spack.build_environment.modifications_from_dependencies(
             spec, context='run'
         )
     )
 
     # Package specific modifications
-    build_env.set_module_variables_for_package(spec.package)
+    spack.build_environment.set_module_variables_for_package(spec.package)
     spec.package.setup_run_environment(env)
 
     return env
diff --git a/lib/spack/spack/util/parallel.py b/lib/spack/spack/util/parallel.py
new file mode 100644
index 0000000000..04562a1801
--- /dev/null
+++ b/lib/spack/spack/util/parallel.py
@@ -0,0 +1,129 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+from __future__ import print_function
+
+import contextlib
+import multiprocessing
+import os
+import sys
+import traceback
+
+import six
+
+from .cpus import cpus_available
+
+
+class ErrorFromWorker(object):
+    """Wrapper class to report an error from a worker process"""
+    def __init__(self, exc_cls, exc, tb):
+        """Create an error object from an exception raised from
+        the worker process.
+
+        The attributes of the process error objects are all strings
+        as they are easy to send over a pipe.
+
+        Args:
+            exc: exception raised from the worker process
+        """
+        self.pid = os.getpid()
+        self.error_message = ''.join(traceback.format_exception(exc_cls, exc, tb))
+
+    def __str__(self):
+        msg = "[PID={0.pid}] {0.error_message}"
+        return msg.format(self)
+
+
+class Task(object):
+    """Wrapped task that trap every Exception and return it as an
+    ErrorFromWorker object.
+
+    We are using a wrapper class instead of a decorator since the class
+    is pickleable, while a decorator with an inner closure is not.
+    """
+    def __init__(self, func):
+        self.func = func
+
+    def __call__(self, *args, **kwargs):
+        try:
+            value = self.func(*args, **kwargs)
+        except Exception:
+            value = ErrorFromWorker(*sys.exc_info())
+        return value
+
+
+def raise_if_errors(*results):
+    """Analyze results from worker Processes to search for ErrorFromWorker
+    objects. If found print all of them and raise an exception.
+
+    Args:
+        *results: results from worker processes
+
+    Raise:
+        RuntimeError: if ErrorFromWorker objects are in the results
+    """
+    err_stream = six.StringIO()  # sys.stderr
+    errors = [x for x in results if isinstance(x, ErrorFromWorker)]
+    if not errors:
+        return
+
+    # Report the errors and then raise
+    for error in errors:
+        print(error, file=err_stream)
+
+    print('[PARENT PROCESS]:', file=err_stream)
+    traceback.print_stack(file=err_stream)
+    error_msg = 'errors occurred in worker processes:\n{0}'
+    raise RuntimeError(error_msg.format(err_stream.getvalue()))
+
+
+@contextlib.contextmanager
+def pool(*args, **kwargs):
+    """Context manager to start and terminate a pool of processes, similar to the
+    default one provided in Python 3.X
+
+    Arguments are forwarded to the multiprocessing.Pool.__init__ method.
+    """
+    try:
+        p = multiprocessing.Pool(*args, **kwargs)
+        yield p
+    finally:
+        p.terminate()
+        p.join()
+
+
+def num_processes(max_processes=None):
+    """Return the number of processes in a pool.
+
+    Currently the function return the minimum between the maximum number
+    of processes and the cpus available.
+
+    When a maximum number of processes is not specified return the cpus available.
+
+    Args:
+        max_processes (int or None): maximum number of processes allowed
+    """
+    max_processes or cpus_available()
+    return min(cpus_available(), max_processes)
+
+
+def parallel_map(func, arguments, max_processes=None):
+    """Map a task object to the list of arguments, return the list of results.
+
+    Args:
+        func (Task): user defined task object
+        arguments (list): list of arguments for the task
+        max_processes (int or None): maximum number of processes allowed
+
+    Raises:
+        RuntimeError: if any error occurred in the worker processes
+    """
+    task_wrapper = Task(func)
+    if sys.platform != 'darwin':
+        with pool(processes=num_processes(max_processes=max_processes)) as p:
+            results = p.map(task_wrapper, arguments)
+    else:
+        results = list(map(task_wrapper, arguments))
+    raise_if_errors(*results)
+    return results
diff --git a/var/spack/repos/builtin.mock/packages/flake8/package.py b/var/spack/repos/builtin.mock/packages/flake8/package.py
index 3132feaee5..9442124f60 100644
--- a/var/spack/repos/builtin.mock/packages/flake8/package.py
+++ b/var/spack/repos/builtin.mock/packages/flake8/package.py
@@ -39,7 +39,7 @@ class Flake8(Package):
     variant('super-awesome-feature',    default=True,  description='Enable super awesome feature')
     variant('somewhat-awesome-feature', default=False, description='Enable somewhat awesome feature')
 
-    provides('lapack', when='@2.0+super-awesome-feature+somewhat-awesome-feature')
+    provides('somevirt', when='@2.0+super-awesome-feature+somewhat-awesome-feature')
 
     extends('python', ignore='bin/(why|does|every|package|that|depends|on|numpy|need|to|copy|f2py3?)')