Re: memory managment in servers?

Replying to my questions,

>>>>> "Dave" == Dave Hudson <dave@humbug.demon.co.uk> writes:

    Dave> Hi, Basile STARYNKEVITCH wrote:
>> 0. How does the swapping actually work. I believe i begin to
>> understand the filesystem protocol designed for that (ie
>> FS_ABSREAD FS_ABSWRITE and FS_FID) but i don't know how it is
>> used?

    Dave> Erm, well at the moment the swapper doesn't work properly
    Dave> :-( (the principal's right though). The FS_ABSXXXX messages
    Dave> are used simply to acheive the same effect as FS_SEEK
    Dave> followed by either FS_READ or FS_WRITE. In fact none of the
    Dave> filesystem servers can be used as a swap device as swap
    Dave> devices must work as DMA servers - so far only wd, fd and
    Dave> cam can do this. FS_FID is used for handling file mapping
    Dave> and isn't used for swapping.

Thanks for your explanation. I don't know what DMA servers are (i know
what is DMA) but i can leave this question unanswered.

>> 3. I wish and am prepared to have some kind of memory-low
>> message. Does such a beast exists? I mean that my ext2fs
>> allocate dynamically (block and inode) cache and free them on a
>> age basis (ie it free the oldest unused block and inode in
>> server memory -not on disk). So it is very easy to me to handle
>> a PLEASE_GIVE_BACK(N Kbytes) message asking to free at least N
>> kilobytes - i could also handle a simpler MEMORY_LOW message
>> asking to free as much memory as i can.

    Dave> This sounds a good idea. I'd been thinking about how to do
    Dave> this a while back and this seems much saner than my original
    Dave> thoughts :-) I assume that you're proposing this be a
    Dave> message, eg FS_MEMORY, so that we can ask any server that
    Dave> want's too listen for some more memory. I guess the
    Dave> question must be when and how such a message would be sent?
    Dave> I also wonder about how we'd decide which server to ask for
    Dave> memory - I don't know what the answer is though.

Yes. Of course, i meant a server able to give memory back (not wanting
more memory -this is just malloc- but capable of free-ing some
unimportant memory like buffer caches). I believe a FIFO discipline is
enough -the kernel would send some FS_MEMORY messages until it has
enough memory (or perhaps it could be a memory server?? which also
implements malloc?? -or an mmap or sbrk primitive for allocating
virtual|real memory- i'm not clear on that point). FS_MEMORY would be
sent in turn to all servers until there is enough memory. I don't know
how this FS_MEMORY (that i called MEMORY_LOW before) would be
implemented.

-- 
Basile STARYNKEVITCH   ----  Commissariat a l Energie Atomique
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