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Re: crashes due to setting max_parallel_workers=0 - Mailing list pgsql-hackers

From Rushabh Lathia
Subject Re: crashes due to setting max_parallel_workers=0
Date
Msg-id CAGPqQf2C6W8aNy9v9ZW2fJs+7p0MoBFfHEtXG8yypimGXTRUvw@mail.gmail.com
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In response to crashes due to setting max_parallel_workers=0  (Tomas Vondra <tomas.vondra@2ndquadrant.com>)
Responses Re: crashes due to setting max_parallel_workers=0  (David Rowley <david.rowley@2ndquadrant.com>)
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Thanks Tomas for reporting issue and Thanks David for working
on this.

I can see the problem in GatherMerge, specially when nworkers_launched 
is zero. I will look into this issue and will post a fix for the same.

Also another point which I think we should fix is, when someone set 
max_parallel_workers = 0, we should also set the max_parallel_workers_per_gather
to zero. So that way it we can avoid generating the gather path with
max_parallel_worker = 0.

Thanks,


On Sat, Mar 25, 2017 at 2:21 PM, David Rowley <david.rowley@2ndquadrant.com> wrote:
On 25 March 2017 at 13:10, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
> while working on a patch I ran into some crashes that seem to be caused by
> inconsistent handling of max_parallel_workers - queries still seem to be
> planned with parallel plans enabled, but then crash at execution.
>
> The attached script reproduces the issue on a simple query, causing crashed
> in GatherMerge, but I assume the issue is more general.

I had a look at this and found a bunch of off by 1 errors in the code.

I've attached a couple of patches, one is the minimal fix, and one
more complete one.

In the more complete one I ended up ditching the
GatherMergeState->nreaders altogether. It was always the same as
nworkers_launched anyway, so I saw no point in it.
Here I've attempted to make the code a bit more understandable, to
prevent further confusion about how many elements are in each array.
Probably there's more that can be done here. I see GatherState has
nreaders too, but I've not looked into the detail of if it suffers
from the same weirdness of nreaders always matching nworkers_launched.


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 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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Rushabh Lathia

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