Re: Why are memory-mapped storages read-only?

FYI, this is implemented now. I have changed the storage-API
a bit: You now need to decide what kind of storage you want
when you create it. The storage kind may not be changed later.


? ^Anew-storage
Search Results for: new-storage
   1. (new-storage <et>)
   2. (new-storage/managed <et> <n> [<init>])
   3. (new-storage/mmap <et> <file> [<offs> [<readonly>]])
   4. (new-storage/foreign <et> <n> <p> [<readonly>])
choice> 3

Each "constructor function" and what it creates:

<new-storage>         : an empty storage,
<new-storage/managed> : a storage object for memory managed by lush
<new-storage/mmap>    : a storage associated with a file, and
<new-storage/foreign> : a storage associated with memory managed
                         by someone else (e.g., a 3rd-party library)

An empty storage may be associated with "managed memory" only
(using <storage-alloc>).


------------------------------------------------------------------------
(new-storage/mmap <et> <file> [<offs> [<readonly>]])         (storage.c)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* See: storage-readonlyp
Map file into memory and associate that memory with a storage with
element-type et ; return the storage.

File may be a file descriptor or a string containing a filename. Each
element of the file starting at byte offs is made accessible through the
corresponding element of the storage srg . Storages of storage class
AtomStorage or MptrStorage cannot be memory mapped. Mapped storages are
readonly by default (pass () as fourth argument to create a writeable
mmapped storage).


= ()
?




On Wed, 2 Sep 2009, Yann LeCun wrote:

> The mmap functionality was implemented to allow access to
> large data files without having to explicitely load them
> into RAM in advance. Back in the old days of Solaris, if
> you mmapped in read-only mode, the swapping system was
> smart enough to never swap out. Hence you would save
> a lot on disk (or network) bandwidth.
>
> Also, you really don't want a random bug in a Lush program
> to inadvertently corrupt your dataset file on disk......
>
> That said, we could have two types of mmapped storages:
> one for read only, and one for read/write.
>
>  -- Yann
>
> Leon might remember. I seem to rememberthink writing to mmapped storages
> used to crash on Solaris
>
> On Wednesday 02 September 2009, Ralf Juengling wrote:
>> Hi,
>>
>> Does anyone remember why lush does not allow to
>> manipulate memory mapped storages?
>>
>> Ralf
>>
>>
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>
>

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