[PATCH 0/5] Hand over key lookup in a page to vinyl reader thread

[Vladimir Davydov]

On Wed, May 29, 2019 at 06:12:46PM +0300, Vladimir Davydov wrote:
> Vladimir Davydov (5):
>   vinyl: factor out function to lookup key in page
>   vinyl: pass page info by reference to reader thread
>   vinyl: encapsulate reader thread selection logic in a helper function
>   vinyl: do not allow to cancel a fiber reading a page
>   vinyl: lookup key in reader thread

Pushed to master after adding fiber_testcancel and rewriting the comment
to patch 4 as follows:

    vinyl: do not allow to cancel a fiber reading a page
    
    To handle fiber cancellation during page read we need to pin all objects
    referenced by vy_page_read_task. Currently, there's the only such
    object, vy_run. It has reference counting so pinning it is trivial.
    However, to move page lookup to a reader thread, we need to also
    reference key def, tuple format, and key. Format and key have reference
    counting, but key def doesn't - we typically copy it. Copying it in this
    case is too heavy.
    
    Actually, cancelling a fiber manually or on timeout while it's reading
    disk doesn't make much sense with PCIE attached flash drives. It used to
    be reasonable with rotating disks, since a rotating disk controller
    could retry reading a block indefinitely on read failure. It is still
    relevant to Network Attached Storage. On the other hand, NAS has never
    been tested, and what isn't tested, can and should be removed. For
    complex SQL queries we'll be forced to rethink timeout handling anyway.
    
    That being said, let's simply drop this functionality.