Date: Thu, 24 Dec 2009 09:20:28 +0800 From: David Xu <davidxu@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: threads@freebsd.org, freebsd-threads@freebsd.org Subject: Re: first patch for process-shared semaphore Message-ID: <4B32C1DC.9080308@freebsd.org> In-Reply-To: <200912230936.35998.jhb@freebsd.org> References: <4B317741.8080004@freebsd.org> <200912230936.35998.jhb@freebsd.org>
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John Baldwin wrote: > On Tuesday 22 December 2009 8:49:53 pm David Xu wrote: >> This is my first attempt to make process-shared mutex work, this means >> you can mmap(MAP_SHARED) a memory area, and put semaphore there, >> or you can sem_open a named semaphore, and just use it between >> processes, the named semaphore uses file system and mmap(), directory >> /tmp/.semaphore is used as IPC directory, any named semaphore >> locates in the directory. old semaphore implementation still exists >> to make it binary compatible, it uses symbol version. >> >> http://people.freebsd.org/~davidxu/patch/shared_semaphore_1.patch > > I would suggest that you leave named semaphores as they currently exist and > follow this approach instead: > > 1) Named semaphores use ksem_*() still. > 2) sem_init/sem_destroy operate on UTMX-backed semaphores identical to the > ones used in the current libthr code. The semid_t structure now becomes the > full structure that libthr currently allocates with a flag to indicate if it > is a "system" semaphore or otherwise. The pshared flag passed to sem_init() > can be used to set the sharing properties of the UMTX. > 3) All of sem_init/sem_destroy is just in libc. Just move the libthr > implementation bits into libc. > ksem base shared semaphore is slow because whenever you call sem_wait(), it always enters kernel even if count is non-zero, sem_post() also always enters kernel even if there is no waiter. but the new implementation is as simple as just an atomic operation in these cases, I know another competitor OS is doing things in this way.
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