notes.html

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non thread-safe functions (in a traditional sense) can be safely used with
the State Threads.  The library's mutex facilities are practically useless
for a correctly written application (no blocking functions in critical
section) and are provided mostly for completeness.  This absence of locking
greatly simplifies an application design and provides a foundation for
scalability.
<P>

<A NAME="inter">
<H3>Inter-Process Synchronization</H3>
The State Threads library makes it possible to multiplex a large number
of simultaneous connections onto a much smaller number of separate 
processes, where each process uses a many-to-one user-level threading
implementation (<B>N</B> of <B>M:1</B> mappings rather than one <B>M:N</B>
mapping used in native threading libraries on some platforms). This design
is key to the application's scalability.  One can think about it as if a
set of all threads is partitioned into separate groups (processes) where
each group has a separate pool of resources (virtual address space, file
descriptors, etc.).  An application designer has full control of how many
groups (processes) an application creates and what resources, if any,
are shared among different groups via standard UNIX inter-process
communication (IPC) facilities.<P>
There are several reasons for creating multiple processes:
<P>
<UL>
<LI>To take advantage of multiple hardware entities (CPUs, disks, etc.)
available in the system (hardware parallelism).</LI>
<P>
<LI>To reduce risk of losing a large number of user connections when one of
the processes crashes. For example, if <B>C</B> user connections (threads)
are multiplexed onto <B>P</B> processes and one of the processes crashes,
only a fraction (<B>C/P</B>) of all connections will be lost.</LI>
<P>
<LI>To overcome per-process resource limitations imposed by the OS.  For
example, if <TT>select(2)</TT> is used for event polling, the number of
simultaneous connections (threads) per process is
limited by the <TT>FD_SETSIZE</TT> parameter (see <TT>select(2)</TT>).
If <TT>FD_SETSIZE</TT> is equal to 1024 and each connection needs one file
descriptor, then an application should create 10 processes to support 10,000
simultaneous connections.</LI>
</UL>
<P>
Ideally all user sessions are completely independent, so there is no need for
inter-process communication.  It is always better to have several separate
smaller process-specific resources (e.g., data caches) than to have one large
resource shared (and modified) by all processes.  Sometimes, however, there
is a need to share a common resource among different processes.  In that case,
standard UNIX IPC facilities can be used.  In addition to that, there is a way
to synchronize different processes so that only the thread accessing the
shared resource will be suspended (but not the entire process) if that resource
is unavailable.  In the following code fragment a pipe is used as a counting
semaphore for inter-process synchronization:
<PRE>
#ifndef PIPE_BUF
#define PIPE_BUF 512  /* POSIX */
#endif

/* Semaphore data structure */
typedef struct ipc_sem {
  st_netfd_t rdfd;  /* read descriptor */
  st_netfd_t wrfd;  /* write descriptor */
} ipc_sem_t;

/* Create and initialize the semaphore. Should be called before fork(2). */
/* 'value' must be less than PIPE_BUF. */
/* If 'value' is 1, the semaphore works as mutex. */
ipc_sem_t *ipc_sem_create(int value)
{
  ipc_sem_t *sem;
  int p[2];
  char b[PIPE_BUF];

  /* Error checking is omitted for clarity */
  sem = malloc(sizeof(ipc_sem_t));

  /* Create the pipe */
  pipe(p);
  sem->rdfd = st_netfd_open(p[0]);
  sem->wrfd = st_netfd_open(p[1]);

  /* Initialize the semaphore: put 'value' bytes into the pipe */
  write(p[1], b, value);

  return sem;
}

/* Try to decrement the "value" of the semaphore. */
/* If "value" is 0, the calling thread blocks on the semaphore. */
int ipc_sem_wait(ipc_sem_t *sem)
{
  char c;

  /* Read one byte from the pipe */
  if (st_read(sem->rdfd, &c, 1, ST_UTIME_NO_TIMEOUT) != 1)
    return -1;

  return 0;
}

/* Increment the "value" of the semaphore. */
int ipc_sem_post(ipc_sem_t *sem)
{
  char c;

  if (st_write(sem->wrfd, &c, 1, ST_UTIME_NO_TIMEOUT) != 1)
    return -1;

  return 0;
}

</PRE>
<P>

Generally, the following steps should be followed when writing an application
using the State Threads library:
<P>
<OL>
<LI>Initialize the library (<TT>st_init()</TT>).</LI>
<P>
<LI>Create resources that will be shared among different processes:
    create and bind listening sockets, create shared memory segments, IPC
    channels, synchronization primitives, etc.</LI>
<P>
<LI>Create several processes (<TT>fork(2)</TT>). The parent process should
    either exit or become a "watchdog" (e.g., it starts a new process when
    an existing one crashes, does a cleanup upon application termination,
    etc.).</LI>
<P>
<LI>In each child process create a pool of threads
    (<TT>st_thread_create()</TT>) to handle user connections.</LI>
</OL>
<P>

<A NAME="nonnet">
<H3>Non-Network I/O</H3>

The State Threads architecture uses non-blocking I/O on
<TT>st_netfd_t</TT> objects for concurrent processing of multiple user
connections.  This architecture has a drawback:  the entire process and
all its threads may block for the duration of a <I>disk</I> or other
non-network I/O operation, whether through State Threads I/O functions,
direct system calls, or standard I/O functions.  (This is applicable
mostly to disk <I>reads</I>; disk <I>writes</I> are usually performed
asynchronously -- data goes to the buffer cache to be written to disk
later.)  Fortunately, disk I/O (unlike network I/O) usually takes a
finite and predictable amount of time, but this may not be true for
special devices or user input devices (including stdin).  Nevertheless,
such I/O reduces throughput of the system and increases response times.
There are several ways to design an application to overcome this
drawback:

<P>
<UL>
<LI>Create several identical main processes as described above (symmetric
    architecture).  This will improve CPU utilization and thus improve the
    overall throughput of the system.</LI>
<P>
<LI>Create multiple "helper" processes in addition to the main process that
    will handle blocking I/O operations (asymmetric architecture).
    This approach was suggested for Web servers in a
    <A HREF="http://www.cs.rice.edu/~vivek/flash99/">paper</A> by Peter
    Druschel et al. In this architecture the main process communicates with
    a helper process via an IPC channel (<TT>pipe(2), socketpair(2)</TT>).
    The main process instructs a helper to perform the potentially blocking
    operation.  Once the operation completes, the helper returns a
    notification via IPC.
</UL>
<P>

<A NAME="timeouts">
<H3>Timeouts</H3>

The <TT>timeout</TT> parameter to <TT>st_cond_timedwait()</TT> and the
I/O functions, and the arguments to <TT>st_sleep()</TT> and
<TT>st_usleep()</TT> specify a maximum time to wait <I>since the last
context switch</I> not since the beginning of the function call.

<P>The State Threads' time resolution is actually the time interval
between context switches.  That time interval may be large in some
situations, for example, when a single thread does a lot of work
continuously.  Note that a steady, uninterrupted stream of network I/O
qualifies for this description; a context switch occurs only when a
thread blocks.

<P>If a specified I/O timeout is less than the time interval between
context switches the function may return with a timeout error before
that amount of time has elapsed since the beginning of the function
call.  For example, if eight milliseconds have passed since the last
context switch and an I/O function with a timeout of 10 milliseconds
blocks, causing a switch, the call may return with a timeout error as
little as two milliseconds after it was called.  (On Linux,
<TT>select()</TT>'s timeout is an <I>upper</I> bound on the amount of
time elapsed before select returns.)  Similarly, if 12 ms have passed
already, the function may return immediately.

<P>In almost all cases I/O timeouts should be used only for detecting a
broken network connection or for preventing a peer from holding an idle
connection for too long.  Therefore for most applications realistic I/O
timeouts should be on the order of seconds.  Furthermore, there's
probably no point in retrying operations that time out.  Rather than
retrying simply use a larger timeout in the first place.

<P>The largest valid timeout value is platform-dependent and may be
significantly less than <TT>INT_MAX</TT> seconds for <TT>select()</TT>
or <TT>INT_MAX</TT> milliseconds for <TT>poll()</TT>.  Generally, you
should not use timeouts exceeding several hours.  Use
<tt>ST_UTIME_NO_TIMEOUT</tt> (<tt>-1</tt>) as a special value to
indicate infinite timeout or indefinite sleep.  Use
<tt>ST_UTIME_NO_WAIT</tt> (<tt>0</tt>) to indicate no waiting at all.

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