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SubjectRe: [PATCH 2 of 4] Introduce i386 fibril scheduling
On Fri, 2 Feb 2007, Davide Libenzi wrote:

> On Fri, 2 Feb 2007, Ingo Molnar wrote:
> > in fact an app could also /trigger/ the execution of a syscall in a
> > different context - to create parallelism artificially - without any
> > blocking event. So we could do:
> >
> > cookie1 = sys_async(sys_read, params);
> > cookie2 = sys_async(sys_write, params);
> >
> > [ ... calculation loop ... ]
> >
> > wait_on_async_syscall(cookie1);
> > wait_on_async_syscall(cookie2);
> >
> > or something like that. Without user-space having to create threads
> > itself, etc. So basically, we'd make kernel threads more useful, and
> > we'd make threading safer - by only letting syscalls thread.
> Since I still think that the many-thousands potential async operations
> coming from network sockets are better handled with a classical event
> machanism [1], and since smooth integration of new async syscall into the
> standard POSIX infrastructure is IMO a huge win, I think we need to have a
> "bridge" to allow async completions being detectable through a pollable
> (by the mean of select/poll/epoll whatever) device.
> In that way you can handle async operations with the best mechanism that
> is fit for them, and gather them in a single async scheduler.

To clarify further, below are the API and the use case of my userspace
implementation. The guasi_fd() gives you back a pollable (POLLIN) fd to be
integrated in your prefered event retrieval interface. Once it fd is
signaled, you can fetch your completed requests using guasi_fetch() and
schedule work based on that.
The GUASI implementation uses pthreads, but it is clear that an in-kernel
async syscall implementation can take wiser decisions, and optimize the
heck out of it (locks, queues, ...).

- Davide

* Example of async pread using GUASI
static long guasi_wrap__pread(void *priv, long const *params) {

return (long) pread((int) params[0], (void *) params[1],
(size_t) params[2], (off_t) params[3]);

guasi_req_t guasi__pread(guasi_t hctx, void *priv, void *asid, int prio,
int fd, void *buf, size_t size, off_t off) {

return guasi_submit(hctx, priv, asid, prio, guasi_wrap__pread, 4,
(long) fd, (long) buf, (long) size, (long) off);

* guasi by Davide Libenzi (generic userspace async syscall implementation)
* Copyright (C) 2003 Davide Libenzi
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* GNU General Public License for more details.
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
* Davide Libenzi <>

#if !defined(_GUASI_H)
#define _GUASI_H



typedef long (*guasi_syscall_t)(void *, long const *);

typedef struct s_guasi { } *guasi_t;
typedef struct s_guasi_req { } *guasi_req_t;

struct guasi_reqinfo {
void *priv; /* Call private data. Passed to guasi_submit */
void *asid; /* Async request ID. Passed to guasi_submit */
long result; /* Return code of "proc" passed to guasi_submit */
long error; /* errno */
int status; /* GUASI_STATUS_* */

guasi_t guasi_create(int min_threads, int max_threads, int max_priority);
void guasi_free(guasi_t hctx);
int guasi_fd(guasi_t hctx);
guasi_req_t guasi_submit(guasi_t hctx, void *priv, void *asid, int prio,
guasi_syscall_t proc, int nparams, ...);
int guasi_fetch(guasi_t hctx, guasi_req_t *reqs, int nreqs);
int guasi_req_info(guasi_req_t hreq, struct guasi_reqinfo *rinf);
void guasi_req_free(guasi_req_t hreq);


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