libpager: use a fixed number of threads

Previously, libpager used an unbounded number of threads to receive
messages from the pager bucket.  It used sequence barriers to execute
the requests to order requests to each object.

The sequence barriers are implemented in seqnos.c.  A server function
uses _pager_wait_for_seqno to wait for its sequence number and
_pager_release_seqno to release it, or it uses _pager_update_seqno to
do both operations in one step.

These sequence barriers divide each server function in three parts: A,
B, and C.  A_i happens "before" the sequence barrier i, B_i happens
"in order", C_i happens "after" the sequence barrier.  This partial
order < has the following properties:

* This order is *per object*.  Requests to different objects are not
  ordered.

* A_i < B_i,  B_i < C_i (due to the structure of the code)

* B_i < B_{i+1} (this is due to the sequence barriers)

* Note that only the B parts are ordered by the sequence numbers, we
  are free to execute C_i and C_{i+1} in any possible order.  The same
  argument applies to the A parts.

The sequence barriers are implemented using a very simple ticket
algorithm.  Every request, even the invalid ones, is processed by a
thread, and waits until the ticket count reaches its seqno, does some
work in-order, then increments the ticket and awakes all threads that
have piled up up to this moment.  All of them except one will then
discover that it's not their turn yet and go to sleep again.

Creating one thread per request has proven to be problematic as
memory_object requests often arrive in large batches.

This patch does two things:

* Use a single thread to receive messages from the port bucket.  All
  incoming request are put into a queue.

* Use a fixed-number of threads (though even one is actually enough)
  to execute the the server functions.  If multiple threads are used,
  a work-delegation mechanism ensures that the per object order < is
  preserved.

For reference, I used the following command to create workloads that
highlight the problem this patch is addressing:

% settrans t .../ext2fs --sync=30 /dev/sd2s1
...
% /usr/bin/time zsh -c 'for ((i=0; i < 1500; i++)); do
  dd if=/dev/zero of=t/src/$i bs=4k count=290 2>/dev/null
  echo -n .
  if ((i % 100 == 0)) ; then echo -n $i; fi
done'

* libpager/queue.h: New file.
* libpager/demuxer.c: Manage a queue of requests received from the
port bucket.
(pager_demuxer): Just decode the server function and enqueue the
request.
(worker_func): New function that consumes and executes the requests
from the queue.
(service_paging_requests): New function.
(pager_start_workers): Likewise.
* libpager/data-request.c: Remove the seqno barriers.
* libpager/data-return.c: Likewise.
* libpager/data-unlock.c: Likewise.
* libpager/chg-compl.c: Likewise.
* libpager/lock-completed.c: Likewise.
* libpager/no-senders.c: Likewise.
* libpager/notify-stubs.c: Likewise.
* libpager/object-init.c: Likewise.
* libpager/object-terminate.c: Likewise.
* libpager/seqnos.c: Remove file.
* libpager/stubs.c: Likewise.
* libpager/pager.h (pager_demuxer): Drop declaration.
(pager_start_workers): New declaration.
* libpager/priv.h: Remove the _pager_seqno declarations.
* libpager/Makefile (SRCS): Drop seqnos.c.
* console/pager.c (user_pager_init): Call pager_start_workers.
* libdiskfs/disk-pager.c: Likewise.
* storeio/pager.c: Likewise.
* ext2fs/pager.c (service_paging_requests): Remove function.
(create_disk_pager): Start separate file pager using
`pager_start_workers'.
* fatfs/pager.c (service_paging_requests): Remove function.
(create_fat_pager): Start separate file pager using
`pager_start_workers'.

1 files changed, 276 insertions, 11 deletions

diff --git a/libpager/demuxer.c b/libpager/demuxer.c
index b4d40541..efdf2853 100644
--- a/libpager/demuxer.c
+++ b/libpager/demuxer.c
@@ -1,5 +1,5 @@
 /* Demuxer for pager library
-   Copyright (C) 1994, 1995, 2002, 2011 Free Software Foundation
+   Copyright (C) 1993-2014 Free Software Foundation
 
    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License as
@@ -15,26 +15,291 @@
    along with this program; if not, write to the Free Software
    Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */
 
+#include <error.h>
+#include <mach/mig_errors.h>
+#include <pthread.h>
+#include <string.h>
+
 #include "priv.h"
 #include "memory_object_S.h"
 #include "notify_S.h"
+#include "queue.h"
+
+/*
+  Worker pool for the server functions.
+
+  A single thread receives messages from the port bucket and puts them
+  into a queue.  A fixed number of consumers actually execute the
+  server functions and send the reply.
+
+  The requests to an object O have to be processed in the order they
+  were received.  To this end, each worker has a local queue and a
+  tag.  If a thread processes a request to O, it sets its tag to a
+  unique identifier representing O.  If another thread now dequeues a
+  second request to O, it enqueues it to the first workers queue.
+
+  At least one worker thread is necessary.
+*/
+#define WORKER_COUNT 1
+
+/* An request contains the message received from the port set.  */
+struct request
+{
+  struct item item;
+  mig_routine_t routine;
+  mach_msg_header_t *inp;
+  mach_msg_header_t *outp;
+};
+
+/* A worker.  */
+struct worker
+{
+  struct requests *requests;	/* our pagers request queue */
+  struct queue queue;	/* other workers may delegate requests to us */
+  unsigned long tag;	/* tag of the object we are working on */
+};
+
+/* This is the queue for incoming requests.  A single thread receives
+   messages from the port set, looks the service routine up, and
+   enqueues the request here.  */
+struct requests
+{
+  struct port_bucket *bucket;
+  struct queue queue;
+  int asleep;
+  pthread_cond_t wakeup;
+  pthread_mutex_t lock;
+  struct worker workers[WORKER_COUNT];
+};
 
 /* Demultiplex a single message directed at a pager port; INP is the
    message received; fill OUTP with the reply.  */
-int
-pager_demuxer (mach_msg_header_t *inp,
+static int
+pager_demuxer (struct requests *requests,
+	       mach_msg_header_t *inp,
 	       mach_msg_header_t *outp)
 {
+  error_t err = MIG_NO_REPLY;
+
+  /* The maximum size of the reply is 2048 bytes.  See the MIG source
+     for details.  */
+  const mach_msg_size_t max_size = 2048;
+
   mig_routine_t routine;
-  if ((routine = _pager_seqnos_memory_object_server_routine (inp)) ||
-      (routine = _pager_seqnos_notify_server_routine (inp)))
+  if (! ((routine = _pager_seqnos_memory_object_server_routine (inp)) ||
+	 (routine = _pager_seqnos_notify_server_routine (inp))))
+    return FALSE;
+
+#define MASK	(8u - 1u)
+  mach_msg_size_t padded_size = (inp->msgh_size + MASK) & ~MASK;
+#undef MASK
+
+  struct request *r = malloc (sizeof *r + padded_size + max_size);
+  if (r == NULL)
+    {
+      err = ENOMEM;
+      goto out;
+    }
+
+  r->routine = routine;
+  r->inp = (mach_msg_header_t *) ((char *) r + sizeof *r);
+  memcpy (r->inp, inp, inp->msgh_size);
+
+  r->outp = (mach_msg_header_t *) ((char *) r + sizeof *r + padded_size);
+  memcpy (r->outp, outp, sizeof *outp);
+
+  pthread_mutex_lock (&requests->lock);
+
+  queue_enqueue (&requests->queue, &r->item);
+
+  /* Awake worker.  */
+  if (requests->asleep > 0)
+      pthread_cond_signal (&requests->wakeup);
+
+  pthread_mutex_unlock (&requests->lock);
+
+  /* A worker thread will reply.  */
+  err = MIG_NO_REPLY;
+
+ out:
+  ((mig_reply_header_t *) outp)->RetCode = err;
+  return TRUE;
+}
+
+/* Consumes requests from the queue.  */
+static void *
+worker_func (void *arg)
+{
+  struct worker *self = (struct worker *) arg;
+  struct requests *requests = self->requests;
+  struct request *r = NULL;
+
+  while (1)
+    {
+      int i;
+      mach_msg_return_t mr;
+
+      /* Free previous message.  */
+      free (r);
+
+      pthread_mutex_lock (&requests->lock);
+
+      /* First, look in our queue for more requests to the object we
+	 have been working on lately.  Some other thread might have
+	 delegated them to us.  */
+      r = queue_dequeue (&self->queue);
+      if (r != NULL)
+	goto got_one;
+
+      /* Nope.  Clear our tag and...  */
+      self->tag = 0;
+
+    get_request_locked:
+      /* ... get a request from the global queue instead.  */
+      while ((r = queue_dequeue (&requests->queue)) == NULL)
+	{
+	  requests->asleep += 1;
+	  pthread_cond_wait (&requests->wakeup, &requests->lock);
+	  requests->asleep -= 1;
+	}
+
+      for (i = 0; i < WORKER_COUNT; i++)
+	if (requests->workers[i].tag
+	    == (unsigned long) r->inp->msgh_local_port)
+	  {
+	    /* Some other thread is working on that object.  Delegate
+	       the request to that worker.  */
+	    queue_enqueue (&requests->workers[i].queue, &r->item);
+	    goto get_request_locked;
+	  }
+
+      /* Claim responsibility for this object by setting our tag.  */
+      self->tag = (unsigned long) r->inp->msgh_local_port;
+
+    got_one:
+      pthread_mutex_unlock (&requests->lock);
+
+      /* Call the server routine.  */
+      (*r->routine) (r->inp, r->outp);
+
+      /* What follows is basically the second part of
+	 mach_msg_server_timeout.  */
+      mig_reply_header_t *request = (mig_reply_header_t *) r->inp;
+      mig_reply_header_t *reply = (mig_reply_header_t *) r->outp;
+
+      switch (reply->RetCode)
+	{
+	case KERN_SUCCESS:
+	  /* Hunky dory.  */
+	  break;
+
+	case MIG_NO_REPLY:
+	  /* The server function wanted no reply sent.
+	     Loop for another request.	*/
+	  continue;
+
+	default:
+	  /* Some error; destroy the request message to release any
+	     port rights or VM it holds.  Don't destroy the reply port
+	     right, so we can send an error message.  */
+	  request->Head.msgh_remote_port = MACH_PORT_NULL;
+	  mach_msg_destroy (&request->Head);
+	  break;
+	}
+
+      if (reply->Head.msgh_remote_port == MACH_PORT_NULL)
+	{
+	  /* No reply port, so destroy the reply.  */
+	  if (reply->Head.msgh_bits & MACH_MSGH_BITS_COMPLEX)
+	    mach_msg_destroy (&reply->Head);
+	  continue;
+	}
+
+      /* Send the reply.  */
+      mr = mach_msg (&reply->Head,
+		     MACH_SEND_MSG,
+		     reply->Head.msgh_size,
+		     0,
+		     MACH_PORT_NULL,
+		     0,
+		     MACH_PORT_NULL);
+
+      switch (mr)
+	{
+	case MACH_SEND_INVALID_DEST:
+	  /* The reply can't be delivered, so destroy it.  This error
+	     indicates only that the requester went away, so we
+	     continue and get the next request.  */
+	  mach_msg_destroy (&reply->Head);
+	  break;
+
+	default:
+	  /* Some other form of lossage; there is not much we can
+	     do here.  */
+	  error (0, mr, "mach_msg");
+	}
+    }
+
+  /* Not reached.  */
+  return NULL;
+}
+
+/* A top-level function for the paging thread that just services paging
+   requests.  */
+static void *
+service_paging_requests (void *arg)
+{
+  struct requests *requests = arg;
+
+  int demuxer (mach_msg_header_t *inp,
+	       mach_msg_header_t *outp)
+  {
+    return pager_demuxer (requests, inp, outp);
+  }
+
+  ports_manage_port_operations_one_thread (requests->bucket,
+					   demuxer,
+					   0);
+  /* Not reached.  */
+  return NULL;
+}
+
+/* Start the worker threads libpager uses to service requests.  */
+error_t
+pager_start_workers (struct port_bucket *pager_bucket)
+{
+  error_t err;
+  int i;
+  pthread_t t;
+  struct requests *requests;
+
+  requests = malloc (sizeof *requests);
+  if (requests == NULL)
+    return ENOMEM;
+
+  requests->bucket = pager_bucket;
+  requests->asleep = 0;
+  queue_init (&requests->queue);
+  pthread_cond_init (&requests->wakeup, NULL);
+  pthread_mutex_init (&requests->lock, NULL);
+
+  /* Make a thread to service paging requests.  */
+  err = pthread_create (&t, NULL, service_paging_requests, requests);
+  if (err)
+    return err;
+  pthread_detach (t);
+
+  for (i = 0; i < WORKER_COUNT; i++)
     {
-      (*routine) (inp, outp);
-      return TRUE;
+      requests->workers[i].requests = requests;
+      requests->workers[i].tag = 0;
+      queue_init (&requests->workers[i].queue);
+
+      err = pthread_create (&t, NULL, &worker_func, &requests->workers[i]);
+      if (err)
+	return err;
+      pthread_detach (t);
     }
 
-  /* Synchronize our bookkeeping of the port's seqno with the one
-     consumed by this bogus message.  */
-  _pager_update_seqno (inp->msgh_local_port, inp->msgh_seqno);
-  return FALSE;
+  return err;
 }