diff --git a/HISTORY.rst b/HISTORY.rst
index 5a966763..7c336f37 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -3,9 +3,10 @@
 History
 -------
 
-v0.18.5 (unreleased)
+v0.19.0 (unreleased)
 ....................
-* Python 3.8 support
+* Python 3.8 support, #178
+* fix concurrency with multiple workers, #180
 
 v0.18.4 (2019-12-19)
 ....................
diff --git a/arq/version.py b/arq/version.py
index 2f5683b6..731728a3 100644
--- a/arq/version.py
+++ b/arq/version.py
@@ -1,3 +1,3 @@
 __all__ = ('VERSION',)
 
-VERSION = '0.18.5a1'
+VERSION = '0.19a1'
diff --git a/arq/worker.py b/arq/worker.py
index be8afc7b..af7f0cf0 100644
--- a/arq/worker.py
+++ b/arq/worker.py
@@ -193,7 +193,7 @@ def __init__(
         self.job_timeout_s = to_seconds(job_timeout)
         self.keep_result_s = to_seconds(keep_result)
         self.poll_delay_s = to_seconds(poll_delay)
-        self.queue_read_limit = queue_read_limit or max_jobs
+        self.queue_read_limit = queue_read_limit or max(max_jobs * 5, 100)
         self._queue_read_offset = 0
         self.max_tries = max_tries
         self.health_check_interval = to_seconds(health_check_interval)
@@ -286,7 +286,11 @@ async def main(self):
                     await asyncio.gather(*self.tasks)
                     return
 
-    async def _poll_iteration(self):
+    async def _poll_iteration(self) -> None:
+        """
+        Get ids of pending jobs from the main queue sorted-set data structure and start those jobs, remove
+        any finished tasks from self.tasks.
+        """
         count = self.queue_read_limit
         if self.burst and self.max_burst_jobs >= 0:
             burst_jobs_remaining = self.max_burst_jobs - self._jobs_started()
@@ -299,7 +303,8 @@ async def _poll_iteration(self):
             job_ids = await self.pool.zrangebyscore(
                 self.queue_name, offset=self._queue_read_offset, count=count, max=now
             )
-        await self.run_jobs(job_ids)
+
+        await self.start_jobs(job_ids)
 
         for t in self.tasks:
             if t.done():
@@ -309,7 +314,10 @@ async def _poll_iteration(self):
 
         await self.heart_beat()
 
-    async def run_jobs(self, job_ids):
+    async def start_jobs(self, job_ids: List[str]) -> None:
+        """
+        For each job id, get the job definition, check it's not running and start it in a task
+        """
         for job_id in job_ids:
             await self.sem.acquire()
             in_progress_key = in_progress_key_prefix + job_id
@@ -323,6 +331,7 @@ async def run_jobs(self, job_ids):
                 if ongoing_exists or not score:
                     # job already started elsewhere, or already finished and removed from queue
                     self.sem.release()
+                    logger.debug('job %s already running elsewhere', job_id)
                     continue
 
                 tr = conn.multi_exec()
diff --git a/tests/test_worker.py b/tests/test_worker.py
index 5a55eae4..d8987664 100644
--- a/tests/test_worker.py
+++ b/tests/test_worker.py
@@ -501,7 +501,8 @@ async def test_queue_read_limit_equals_max_jobs(arq_redis: ArqRedis, worker):
         await arq_redis.enqueue_job('foobar')
 
     assert await arq_redis.zcard(default_queue_name) == 4
-    worker: Worker = worker(functions=[foobar], max_jobs=2)
+    worker: Worker = worker(functions=[foobar], queue_read_limit=2)
+    assert worker.queue_read_limit == 2
     assert worker.jobs_complete == 0
     assert worker.jobs_failed == 0
     assert worker.jobs_retried == 0
@@ -521,6 +522,13 @@ async def test_queue_read_limit_equals_max_jobs(arq_redis: ArqRedis, worker):
     assert worker.jobs_retried == 0
 
 
+async def test_queue_read_limit_calc(worker):
+    assert worker(functions=[foobar], queue_read_limit=2, max_jobs=1).queue_read_limit == 2
+    assert worker(functions=[foobar], queue_read_limit=200, max_jobs=1).queue_read_limit == 200
+    assert worker(functions=[foobar], max_jobs=18).queue_read_limit == 100
+    assert worker(functions=[foobar], max_jobs=22).queue_read_limit == 110
+
+
 async def test_custom_queue_read_limit(arq_redis: ArqRedis, worker):
     for _ in range(4):
         await arq_redis.enqueue_job('foobar')
@@ -686,7 +694,7 @@ async def foo(ctx, v):
     caplog.set_level(logging.DEBUG, logger='arq.worker')
     await arq_redis.enqueue_job('foo', 1, _job_id='testing')
     worker: Worker = worker(functions=[func(foo, name='foo')])
-    await asyncio.gather(*[worker.run_jobs(['testing']) for _ in range(5)])
+    await asyncio.gather(*[worker.start_jobs(['testing']) for _ in range(5)])
     # debug(caplog.text)
     assert 'multi-exec error, job testing already started elsewhere' in caplog.text
     assert 'WatchVariableError' not in caplog.text