Hi, It's not a real `mmap` but more a `read()` without `Lwt`. In the case when we limit the access to the block (only for reading), it's fine to provide a `read()` without a scheduling idea mainly because whatever what we do with the block, it's a read-only block and data will be the same all the time. The idea behind that is to unlock the ability to create a read-only file-system and where the access of datas will not be determined by a underlying scheduler. A new signature like: ```ocaml module type Mirage_block.RD = sig type t
val read : offset:int64 -> Cstruct.t end ``` Will allow us to make a `Mirage_kv.RO` and be able to compose such layout (the file-system layout) with something else (like `ccm_block`). Best, On Wed, Nov 30, 2022 at 6:01 PM Anil Madhavapeddy <av...@cl.cam.ac.uk> wrote: > On 30 Nov 2022, at 15:13, Hannes Mehnert <han...@mehnert.org> wrote: > > > ## `mmap` available on `Mirage_block.S` (dinosaure, > https://github.com/mirage/mirage-block/issues/53) > - dinosaure has an implementation to get a part of the block (similar to > mmap), without being in the Lwt monad > - at the moment, read is in Lwt.t, i.e. does not block, but returns the > filled page(s) > - dinosaure needs a blocking function that returns the data > - the solo5 interface is already blocking (and synchronous), > mirage-block-solo5 adds the asynchronous stuff > - christiano mentions that it could be done with locking > - maybe develop a block read-only interface with a synchronous read > > > In general, having "automatic" scheduling via mmap is a bad idea for > anything non-trivial, since you slow to a crawl when under memory pressure > and having a lot of page faults. There's no way a caller can determine > whether or not a set of accesses will result in a blocking fetch or not. > > It may be workable for a read-only mmap, but... why do you want it? To get > out of Lwt allocations? > > Anil > -- Romain Calascibetta - http://din.osau.re/
