RE: Another Semaphore question.
On Sun, 6 May 2001, Oded Arbel wrote: I don't know of any other really good Linux programming books out there, but if you need something to read to get you started with API programming in Linux, or want a backup sitting on your desktop - I can recommend it. this book sohuld be used not necessarily to _learn_ the API. it should be used when you want to make sure your code is portable to other unices, and that you've covered all cases. it covers things that man pages miss or don't explain clearly, and more then once i've used it to make sure a piece of code is written properly. there are various cases where you write code and it looks like it works, and then you start hitting bugs that cannot be reproduced, and after checking with steven's various books, you find you did something wrong, you fix it - and see those non-reproduceable bugs go away. and in those cases, you only need to read 2-3 pages from the book, so you (hopefully) don't manage to get bored yet. -- guy For world domination - press 1, or dial 0, and please hold, for the creator. -- nob o. dy = To unsubscribe, send mail to [EMAIL PROTECTED] with the word unsubscribe in the message body, e.g., run the command echo unsubscribe | mail [EMAIL PROTECTED]
Re: Another Semaphore question.
Yes. This is one of the problems with semaphores. BTW, why not using file locking instead of it? It has much cleaner and richer semantics and almost none of the problems semaphores have. not terribly good portability, according to /usr/src/linux/Documentatino/mandatory.txt , for mandatory locks? advisory locks can be used, but then you only have a single such lock available per file. if you need to use several locks, you need to keep several files open, and since there's a limit on file descriptors per process, this is not scaleable. as to oded's reply: I don't think its an option I have for the current implementation - it's file IO, and should be much slower, right ? well, i'm not sure that usage of locks performs any file I/O (after all, all the locks data is kept in memory, not on disk, as this is NOT persistent data). however, looking at the kernel's sources, it seems that at least the implementation of mandatory locks would be less efficient then that of semaphores. the locks on a file are kept as a linked list, and sometimes get traversed - thus, if you have many locks active on a single file, that'll be less efficient then using semaphores. as for advisory locks (handled using the flock() syscall) - there is also a linked list of locks kept on a file. however, if you'll only use exclusive locks, there could be only one lock in this list at any given time. and there is no file I/O during those operations. -- guy For world domination - press 1, or dial 0, and please hold, for the creator. -- nob o. dy = To unsubscribe, send mail to [EMAIL PROTECTED] with the word unsubscribe in the message body, e.g., run the command echo unsubscribe | mail [EMAIL PROTECTED]
Re: Another Semaphore question.
On Sun, 6 May 2001, guy keren wrote: I don't think its an option I have for the current implementation - it's file IO, and should be much slower, right ? well, i'm not sure that usage of locks performs any file I/O (after all, all the locks data is kept in memory, not on disk, as this is NOT persistent data). however, looking at the kernel's sources, it seems that at least the implementation of mandatory locks would be less efficient then that of semaphores. the locks on a file are kept as a linked list, and sometimes get traversed - thus, if you have many locks active on a single file, that'll be less efficient then using semaphores. as for advisory locks (handled using the flock() syscall) - there is also a linked list of locks kept on a file. however, if you'll only use exclusive locks, there could be only one lock in this list at any given time. and there is no file I/O during those operations. Thanks. I'll think about, but I don't think I'll change from semaphores just yet. I don't like the idea of creating temp files for locks, and I can't lock against an existing file, since I sometimes need to create several different locks in the same application, and w/o creating files, this mechanism would need to be way too complicated. Oded = To unsubscribe, send mail to [EMAIL PROTECTED] with the word unsubscribe in the message body, e.g., run the command echo unsubscribe | mail [EMAIL PROTECTED]
Re: Another Semaphore question.
gk not terribly good portability, according to gk /usr/src/linux/Documentatino/mandatory.txt , for mandatory locks? gk advisory locks can be used, but then you only have a single such lock gk available per file. if you need to use several locks, you need to keep What? You sure should use advisory ones, for if you thought about semaphores, you are surely writing cooperative application. But advisory lock can be set one per each _byte_ of the file, not one per file. And since you are not limited by the filesize, this gives you, IIRC, about 4G of locks per file. Each lock, though, wastes certain amount of kernel memory, so you should not use all of them ;) gk several files open, and since there's a limit on file descriptors per gk process, this is not scaleable. Frankly, how many locks do you need? Thousands? Billions? Most probably two or three, really. gk well, i'm not sure that usage of locks performs any file I/O (after all, No, it does not. As for speed - this is very OS-dependant. I remember seeing some numbers in Stevens, but I don't remember what they told. IIRC, for Linux it's pretty much the same. gk and sometimes get traversed - thus, if you have many locks active on a gk single file, that'll be less efficient then using semaphores. Well, that's true - but really,, how many apps except databases (where semaphores are irrelevant anyway, since you need to lock real files) have more than ten locks? -- [EMAIL PROTECTED] \/ There shall be counsels taken Stanislav Malyshev /\ Stronger than Morgul-spells phone +972-3-9316425/\ JRRT LotR. http://sharat.co.il/frodo/ whois:!SM8333 = To unsubscribe, send mail to [EMAIL PROTECTED] with the word unsubscribe in the message body, e.g., run the command echo unsubscribe | mail [EMAIL PROTECTED]
Re: Another Semaphore question.
OA I don't think its an option I have for the current OA implementation - it's file IO, and should be much slower, right OA ? Wrong. Record locking has nothing to do with file I/O - it is done entirely in memory. -- [EMAIL PROTECTED] \/ There shall be counsels taken Stanislav Malyshev /\ Stronger than Morgul-spells phone +972-3-9316425/\ JRRT LotR. http://sharat.co.il/frodo/ whois:!SM8333 = To unsubscribe, send mail to [EMAIL PROTECTED] with the word unsubscribe in the message body, e.g., run the command echo unsubscribe | mail [EMAIL PROTECTED]
Re: Another Semaphore question.
OA I'll think about, but I don't think I'll change from semaphores just yet. OA I don't like the idea of creating temp files for locks, and I can't lock Actually, cleaning temp file is by far easier than cleaning semaphore. Especially if your app can be killed any moment. OA against an existing file, since I sometimes need to create several OA different locks in the same application, and w/o creating files, this OA mechanism would need to be way too complicated. Why? Create one file and lock bytes 1, 2 and 3. BTW, semaphores are much more scarce resource than files - they are system-wide and defaults are pretty low on them. -- [EMAIL PROTECTED] \/ There shall be counsels taken Stanislav Malyshev /\ Stronger than Morgul-spells phone +972-3-9316425/\ JRRT LotR. http://sharat.co.il/frodo/ whois:!SM8333 = To unsubscribe, send mail to [EMAIL PROTECTED] with the word unsubscribe in the message body, e.g., run the command echo unsubscribe | mail [EMAIL PROTECTED]
Re: Another Semaphore question.
On 03-May-2001 Stanislav Malyshev a.k.a Frodo wrote: OA and then leave it lying around and exit the process - will it stay OA in the system when no current process uses it ? Yes. This is one of the problems with semaphores. BTW, why not using This is not a PROBLEM but a FEATURE. All Sys-V IPC (semaphores, message queues, shared memory) outlive their creators. You are completely right that if this behavior isn't desired in the application, we should choose other IPC mechanism (that's why we have so many of them). However, semaphore aren't being used just for Mutual-Exclusion problems. A classical counter example is producer-consumer that aren't running continuously. Oron Peled Voice/Fax: +972-4-8228492 [EMAIL PROTECTED] http://www.actcom.co.il/~oron There's nothing wrong with Windows 2000... ...that Linux can't fix = To unsubscribe, send mail to [EMAIL PROTECTED] with the word unsubscribe in the message body, e.g., run the command echo unsubscribe | mail [EMAIL PROTECTED]
Re: Another Semaphore question.
This is not a PROBLEM but a FEATURE. All Sys-V IPC (semaphores, message queues, shared memory) outlive their creators. Wrong. Shared memory can be pre-deleted, so that it is removed when last process is detached. Just like files. Semaphores can not. In fact, there's no good way to insure that application cleans up its semaphores on exit. If you know one, you are more than welcome to share. However, semaphore aren't being used just for Mutual-Exclusion problems. A classical counter example is producer-consumer that aren't running continuously. Well, in fact in most cases sempahores are used for mutual exclusion or usage control (something like does anyone uses this resource or can I mess with it?). From my experience, file locks work way better in this case. -- [EMAIL PROTECTED] \/ There shall be counsels taken Stanislav Malyshev /\ Stronger than Morgul-spells phone +972-3-9316425/\ JRRT LotR. http://sharat.co.il/frodo/ whois:!SM8333 = To unsubscribe, send mail to [EMAIL PROTECTED] with the word unsubscribe in the message body, e.g., run the command echo unsubscribe | mail [EMAIL PROTECTED]
Another Semaphore question.
Hi list. One more semaphore question if you please - if I create a semaphoer set, and then leave it lying around and exit the process - will it stay in the system when no current process uses it ? If so - what does it take to clear it from the system : loging out ? going to runlevel 1 ? rebooting ? Oded = To unsubscribe, send mail to [EMAIL PROTECTED] with the word unsubscribe in the message body, e.g., run the command echo unsubscribe | mail [EMAIL PROTECTED]
Re: Another Semaphore question.
OA One more semaphore question if you please - if I create a semaphoer set, OA and then leave it lying around and exit the process - will it stay in the OA system when no current process uses it ? Yes. This is one of the problems with semaphores. BTW, why not using file locking instead of it? It has much cleaner and richer semantics and almost none of the problems semaphores have. OA If so - what does it take to clear it from the system : loging out ? going OA to runlevel 1 ? rebooting ? Or you do RMID operation on it (btw, you can RMID used semaphore too, so you will need to check if it's in use manually), or reboot. -- [EMAIL PROTECTED] \/ There shall be counsels taken Stanislav Malyshev /\ Stronger than Morgul-spells phone +972-3-9316425/\ JRRT LotR. http://sharat.co.il/frodo/ whois:!SM8333 = To unsubscribe, send mail to [EMAIL PROTECTED] with the word unsubscribe in the message body, e.g., run the command echo unsubscribe | mail [EMAIL PROTECTED]
Re: Another Semaphore question.
Hi list. One more semaphore question if you please - if I create a semaphoer set, and then leave it lying around and exit the process - will it stay in the system when no current process uses it ? If so - what does it take to clear it from the system : loging out ? going to runlevel 1 ? rebooting ? Oded ipcs and then ipcrm ? -- Shaul Karl [EMAIL PROTECTED] = To unsubscribe, send mail to [EMAIL PROTECTED] with the word unsubscribe in the message body, e.g., run the command echo unsubscribe | mail [EMAIL PROTECTED]
