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From Marco de Abreu <marco.g.ab...@gmail.com>
Subject Re: Making new operators and AMP lists
Date Tue, 28 May 2019 22:51:30 GMT
Hi,

I'm generally in favour of these kind of tests since they make developers
aware of changes they have to make which they would usually not be aware
of. We have a similar test for tutorials, for example. Whenever somebody
adds a tutorial, there's a validation that assures that all contraints in
our testing environment are met and that they are properly tied into the
system. This AMP test fits into the same category in my opinion and we
never heard bad feedback about these kind of checks.

What seems to be bothering people is the fact that the feedback time is too
high. Thus, I'd like to propose to move the test into the sanity-test stage
instead of doing it as part of the unit tests which take quite a bit of
time until they're actually executed. The sanity checks run immediately and
give a response within about 1 minute.

While I understand that this might increase the amount of work a developer
has to do if they develop a new operator, I think that this is the right
thing to do. Developers won't know of every single feature other people
worked on and thus might simply miss adding the support for it. This kind
of test on the other hand makes them aware of it. If they'd like to opt
out, it's one single line they would have to change and then they're
totally fine. On the other hand, this might motivate them to add the
support since the kernel would be the last piece and everything else would
already be implemented.

Considering how often a PR gets declined because of linting errors, I'd say
that these kind of errors are WAY more frequent that AMP telling somebody
to add their operator to a list. Considering that this would only have to
be done once per operator, that's work of about one minute. Add that to the
waiting time of the sanity check and you're left with about five "wasted"
minutes.

I'm opposed towards adding a warning or treating them as float32 by default
since the operator author wouldn't notice. What will happen is that people
won't know about AMP and simply forget about low precision in general until
they're actively reminded. This check will remind them actively and thus
bring more attention to the feature. I know that the feature is still
experimental, but we have just started with the 1.6 branch and thus there's
enough time to make the experimental features production ready. Adding this
test early on will allow others to add the support for AMP during the early
stage of the 1.6 branch instead of asking them in the last few weeks before
a release. The result would only be that stuff is rushed or forgotten.

To sum it up: I think this test is good and it should be kept as error, but
it should be moved to sanity checks.

-Marco

On Wed, May 29, 2019 at 12:21 AM Sheng Zha <zhasheng@apache.org> wrote:

> Thanks for initiating the discussion.
>
> The premise for adding the test was to make sure that AMP feature is "not
> broken", but that's IMO not the right view. AMP is not supposed to support
> a new operator it hasn't seen before in the first place. There's no way for
> it to know whether the fp32 cast should happen or not. So AMP feature
> cannot provide the guarantee that it works for all future operators. Thus,
> adding new operators to AMP list should be considered new feature instead
> of fixing existing feature.
>
> The AMP test that breaks upon the addition of new operator is thus
> equivalent to forcing developers of the new operator to add the new support
> for AMP. This feels wrong. Especially given that AMP is an experimental
> feature in contrib namespace (i.e. no semver guarantee), this practice
> should be stopped immediately. We cannot force new developers to invest
> into experimental feature this way.
>
> I'd suggest the following changes:
> - for new operators that aren't registered in AMP, cast to float32 by
> default and print one-time warning. People using AMP who want to avoid
> casting can register it in the AMP's list.
> - change the test to print warning about the operators that are not listed
> so that it's easy to track the problem.
>
> -sz
>
> On 2019/05/28 21:32:42, Przemys��aw Tr��dak <ptrendx@apache.org> wrote:
> > Dear Community,
> >
> > One of the recently merged features of the 1.5 release, AMP (Automatic
> Mixed Precision) support (PR [1], design doc [5]), introduced a requirement
> that every new operator added to MXNet would need to be present in 1 of the
> lists (in [2]). To make sure that this requirement is not broken when
> somebody adds a new operator and does not know about AMP's existence, a
> test was added to CI ([3]).
> >
> > A few people reached out to me (the original author of the feature)
> saying this test increases a burden on a developer of new operators and
> should not be an actual error, but just warning (PR for that change [4]).
> That is why I would like to present a motivation for it and discuss with
> the wider audience why I feel it was necessary.
> >
> > First, for people who do not know the details of what AMP is - it is a
> solution that tries to automatically apply best practices of training in
> lower precision (FP16) to user's FP32 model in order to fully utilize
> capabilities of modern GPUs (and potentially other hardware in the future).
> It does so by casting to lower precision inputs to operators benefitting
> from it, while casting to full precision inputs of operators that are
> unsafe to run in lower precision or just do not support it.
> >
> > The first iteration of AMP kept 2 main lists of operators - operators
> that are beneficial and safe to do in fp16 and operators that need to be
> cast to FP32. The problem (raised in review of the PR [6], [8]) is how to
> make sure that the feature works as intended and is not inadvertently
> broken by somebody adding a new operator. The failure scenario here is
> adding a new operator that does not support FP16 and so should be cast to
> FP32, but AMP does not know about its existence and so does not do the
> casting. The solution proposed in the review was to implicitly treat all of
> the unknown operators as FP32-only and keep the list of operators that work
> fine in both FP16 and FP32. This solution however does not really work,
> because there are multiple operators (most notably optimizers) where
> introducing additional casting of the input to FP32 would break the
> operator.
> >
> > That is why after discussion with a few members of the community, I
> decided to proceed with all lists being explicit and introducing the test
> that would fail when somebody added an operator without classifying it into
> 1 of the categories, and explain clearly how to do it [7]. It is not ideal
> solution, as it introduces some burden on the developers who are not aware
> about AMP, however in the typical case of adding at most a few operators to
> MXNet the inconvenience is I think pretty minor while important for the
> feature correctness going forward.
> >
> > I would like to gather Community feedback and ideas how to handle this
> situation.
> >
> > [1] https://github.com/apache/incubator-mxnet/pull/14173
> > [2]
> https://github.com/apache/incubator-mxnet/blob/master/python/mxnet/contrib/amp/lists/symbol.py
> > [3]
> https://github.com/apache/incubator-mxnet/blob/master/tests/python/unittest/test_amp.py
> > [4] https://github.com/apache/incubator-mxnet/pull/15085
> > [5]
> https://docs.google.com/document/d/1sQzMoPEwux0WXSWirY07us1POD_6Y8pLYq--b9Fvd1o/edit?usp=sharing
> > [6]
> https://github.com/apache/incubator-mxnet/pull/14173#discussion_r270728019
> > [7]
> https://github.com/apache/incubator-mxnet/blob/master/tests/python/unittest/test_amp.py#L62-L80
> > [8]
> https://github.com/apache/incubator-mxnet/pull/14173#pullrequestreview-235846341
> >
>

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