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<H3>Thread: <A HREF="/mail/thread.php3?subject=kern%2F8500%3A+FreeBSD+3.0+thread+scheduler+is+broken&list=145">kern/8500: FreeBSD 3.0 thread scheduler is broken</A></H3><P>
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<TR><TD><PRE>FROM: Peter Wemm<BR>DATE: 10/31/1998�13:10:00<BR>SUBJECT: RE: kern/8500: FreeBSD 3.0 thread scheduler is broken<P>�<P><PRE><FONT FACE="COURIER">The following reply was made to PR kern/8500; it has been noted by GNATS. From: Peter Wemm <<A HREF="/[email protected]_p_a_m"><EMAIL: PROTECTED></A>> To: HighWind Software Information <<A HREF="/[email protected]_p_a_m"><EMAIL: PROTECTED></A>> Cc: <A HREF="/[email protected]_p_a_m"><EMAIL: PROTECTED></A> Subject: Re: kern/8500: FreeBSD 3.0 thread scheduler is broken Date: Sun, 01 Nov 1998 03:08:44 +0800 HighWind Software Information wrote: > > > >Number: 8500 > > >Category: kern > > >Synopsis: FreeBSD 3.0 thread scheduler is broken > > > > >Description: > > When an application has threads that are I/O intensive, that thread > > unfairly steals cycles from all other threads. This makes writing > > an MT program that does any real amount of I/O impossible. > > Yes, open(), read(), write(), etc block the entire process. The libc_r > thread engine only works for things that can be select()ed apon, and read/ > write cannot (yet). > > Ummm. Not to be rude.. But... > > That is NOT TRUE AT ALL. read() and write() CERTAINLY are selected > apon and do NOT block the whole process when using libc_r. > > Read /usr/src/lib/libc_r/uthread/uthread_write.c and see for yourself. Yes, but only if the file descriptor itself supports O_NONBLOCK mode.. /* called by open() wrapper */ _thread_fd_table_init(int fd) { ... /* Get the flags for the file: */ if (fd >= 3 && (entry->flags = _thread_sys_fcntl(fd, F_GETFL, 0)) == -1) { ret = -1; } else { ... } And in write(), it just calls the write system call: write() { .... /* Check if file operations are to block */ blocking = ((_thread_fd_table[fd]->flags & O_NONBLOCK) == 0); /* * Loop while no error occurs and until the expected number * of bytes are written if performing a blocking write: */ while (ret == 0) { /* Perform a non-blocking write syscall: */ ^^^^^^^^^^^^^^^^^ - only if opened in O_NONBLOCK n = _thread_sys_write(fd, buf + num, nbytes - num); /* Check if one or more bytes were written: */ if (n > 0) It`s similar for read(). There`s a couple of big ifs so far. *If* you open the file in O_NONBLOCK mode specifically, then you get non-blocking read/write syscalls. The syscalls themselves are only non-blocking *if* the underlying fd supports it. Sockets and pipes support it. Files (at least on ufs/ffs) do not. No matter whether you ask for O_NONBLOCK or not, you will always get a blocking read/write with disk IO with read and write. > The only alternatives are to use the aio/lio syscalls (which work), or > rfork(). libc_r could probably be modified to use rfork() to have the > read/write/open/close/etc done in parallel. > > I don`t think that is necessary. It is if you want the threading to continue while the disk is grinding away. aio_read() and aio_write() would probably be enough to help file IO, but open() will still be blocking. Squid has some fairly extensive async disk-IO routines. They happen to use pthreads as a mechanism of having child processes do the blocking work. FreeBSD could use rfork() for arranging the blocking stuff in child processes with shared address space. It would be a lot of work though, and would be a problem on SMP systems. > -Rob Cheers, -Peter To Unsubscribe: send mail to <A HREF="/[email protected]_p_a_m"><EMAIL: PROTECTED></A> with "unsubscribe freebsd-bugs" in the body of the message
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