Re: [PATCH BUGFIX V2 0/3] null_blk: fix throughput losses and hangs

[Jens Axboe <axboe@fb.com>]

On 12/01/2015 03:48 AM, Paolo Valente wrote:
> Hi,
> here is an updated version of the patchset, differing from the
> previous version only in that it reinstates the missing extra check
> pointed out in [2]. For your convenience, the content of the cover
> letter for the previous version follows.
>
> While doing some tests with the null_blk device driver, we bumped into
> two problems: first, unjustified and in some cases high throughput
> losses; second, actual hangs. These problems seem to be the
> consequence of the combination of three causes, and this patchset
> introduces a fix for each of these causes. In particular, changes
> address:
> . an apparent flaw in the logic with which delayed completions are
>    implemented: this flaw causes, with unlucky but non-pathological
>    workloads, actual request-completion delays to become arbitrarily
>    larger than the configured delay;
> . the missing restart of the device queue on the completion of a request in
>    single-queue non-delayed mode;
> . the overflow of the request-delay parameter, when extremely high values
>    are used (e.g., to spot bugs).
>
> To avoid possible confusion, we stress that these fixes *do not* have
> anything to do with the problems highlighted in [1] (tests of the
> multiqueue xen-blkfront and xen-blkback modules with null_blk).
>
> You can find more details in the patch descriptions.

Thanks Paolo, added.

-- 
Jens Axboe