Hi Wes, Thanks for your good suggestion, we will think about it.
Hi Jacques, Thanks for your feedback. Please allow me to address some of your concerns: > This change is undesirable as it optimizes one path and makes several others behave in unintended ways. Could you please elaborate more on this? > What happens if a vector with nulls shows up? What happens if a user sets a position to a null value in user code when this flag set? Please note that in addition to the flag, we also have BaseValueVector#nullable to tell if a vector is nullable. So if BaseValueVector#nullable = true, it is OK to set a null in any position. However, if BaseValueVector#nullable = false and the user tries to set null in some position, there must be some bugs in their code. > If the answer to the above questions is the use is an advanced user, then > why can't they just call: > PlatformDependent.getInt(vector.memoryAddress() + position * 4). > Why would we introduce something directly in the vector class for this > specialized use case? If the user is advanced, that short memory access > invocation seems fine to use. IMO, the problems of directly invoking the PlatformDependent APIs include: 1. The validity buffer for the vector is still allocated anyway, so the benefit of reduced memory consumption is lost. 2. The user can only use this trick in their client code. If they need to use our libraries (e.g. flight, adapter, algorithm), they still have to pay the performance price. 3. If we simply skip the validity buffer and manipulate the data buffer directly, the vector will easily end up in an inconsistent state, causing unexpected problems. > The whole idea with Arrow is that if you have a specialized algorithm, you can hand write memory reads and writes because you have predictable memory layout. This sounds reasonable. IMO, supporting non-nullable vectors is not a specialized requirement. Instead, it is a general requirement, and has a wide range of applications, especially in relational data processing. So it deserves some special treatment (I think), given the benefits discussed above. Please think about it. Best, Liya Fan On Thu, Sep 10, 2020 at 11:32 PM Wes McKinney <wesmck...@gmail.com> wrote: > I agree with Jacques here. Perhaps what is needed is an unsafe > non-nullable array accessor layer, then there is no need for flags > etc. We've already been writing a lot of such code in C++ (splitting > between no-nulls and some-nulls paths, see also the BitBlockCounter > stuff we've been doing, is such a thing available in Java yet?) and it > hasn't seemed especially burdensome to me. > > On Thu, Sep 10, 2020 at 9:37 AM Jacques Nadeau <jacq...@apache.org> wrote: > > > > This change is undesirable as it optimizes one path and makes several > > others behave in unintended ways. What happens if a vector with nulls > > shows up? What happens if a user sets a position to a null value in user > > code when this flag set? > > > > If the answer to the above questions is the use is an advanced user, then > > why can't they just call: > > PlatformDependent.getInt(vector.memoryAddress() + position * 4). > > > > Why would we introduce something directly in the vector class for this > > specialized use case? If the user is advanced, that short memory access > > invocation seems fine to use. The whole idea with Arrow is that if you > have > > a specialized algorithm, you can hand write memory reads and writes > because > > you have predictable memory layout. > > > > On Wed, Sep 9, 2020 at 1:11 AM Fan Liya <liya.fa...@gmail.com> wrote: > > > > > Hi all, > > > > > > Thanks a lot for your previous feedback. > > > > > > Now we have made some investigation and prepared an initial PR > supporting > > > the non-nullable IntVector [1], as this represents a common scenario. > > > Some initial observations and conclusions can be made. > > > > > > The basic idea of the PR is to provide a global static final flag > > > (NON_NULLABLE_VECTORS_ENABLED) to enable/disable the new feature. > > > 1. When the flag is enabled, manipulations to the validity buffer > would be > > > by-passed if the vector is non-nullable. > > > 2. When the flag is disabled, the behavior is identical to the original > > > code. > > > > > > *First*, we want to show that the change is small. To support the > > > non-nullable IntVector, we need to change classes IntVector, > > > BaseFixedWidthVector (the superclass of IntVector), and BaseValueVector > > > (the superclass of BaseFixedWidthVector). The amount of changes (lines > of > > > code) to each class is given below > > > > > > Class > > > > > > Additions (#lines) > > > > > > Deletions (#lines) > > > > > > Total # lines > > > > > > IntVector > > > > > > 12 > > > > > > 5 > > > > > > 370 > > > > > > BaseFixedWidthVector > > > > > > 30 > > > > > > 11 > > > > > > 925 > > > > > > BaseValueVector > > > > > > 7 > > > > > > 0 > > > > > > 240 > > > > > > It can be seen that the change is small, relative to the class size. > In > > > addition, to support additional vector types, we only need to change > the > > > sub-classes, and no more need to change the super classes. > > > > > > *Second*, we want to show that the performance improvement is notable. > To > > > see this, we give the performance data of the IntBenchmark (with some > > > benchmarks added in the PR). To make the performance as good as > possible, > > > we enable the ARROW_ENABLE_UNSAFE_MEMORY_ACCESS flag and disable > > > the ARROW_ENABLE_NULL_CHECK_FOR_GET flag. Below, we give the data with > the > > > non-nullable vector flag turned off and on, respectively. > > > > > > > > > > > > > > > > > > > > > > > > > > > *(Vector non-nullable flag off)Benchmark Mode > Cnt > > > Score Error UnitsIntBenchmarks.getInt avgt 5 > > > 384.948 ± 3.336 ns/opIntBenchmarks.isNull avgt 5 > > > 1301.005 ± 54.239 ns/opIntBenchmarks.setIntDirectly avgt 5 > > > 15387.486 ± 555.749 ns/opIntBenchmarks.setWithValueHolder avgt 5 > > > 15251.351 ± 134.286 ns/opIntBenchmarks.setWithWriter avgt 5 > > > 28595.586 ± 932.528 ns/op* > > > > > > *(Vector non-nullable flag on)* > > > > > > > > > > > > > > > *IntBenchmarks.getInt avgt 5 384.630 ± 1.547 > > > ns/opIntBenchmarks.isNull avgt 5 3.004 ± 0.110 > > > ns/opIntBenchmarks.setIntDirectly avgt 5 13511.605 ± 135.455 > > > ns/opIntBenchmarks.setWithValueHolder avgt 5 13035.883 ± 196.081 > > > ns/opIntBenchmarks.setWithWriter avgt 5 24734.825 ± 1603.708 > > > ns/op* > > > > > > For the *getInt *operation, there is little performance difference. > This > > > is because we disable the ARROW_ENABLE_NULL_CHECK_FOR_GET flag, so > > > manipulations to the validity buffer are by-passed, even if the > > > non-nullable vector flag is off. When the > ARROW_ENABLE_NULL_CHECK_FOR_GET > > > is enabled, there is a 72.4% performance improvement gained by turning > on > > > the non-nullable vector flag. > > > > > > For the isNull operation, we see 3 orders of magnitude performance > > > improvements by enabling the non-nullable vector flag. > > > > > > For other operations, we see 12.2%, 13.5% and 14.5% performance > > > improvements by turning on the non-nullable vector flag. > > > > > > So it can be seen that notable performance improvements can be gained > for > > > non-nullable vectors. > > > > > > *Third*, we want to claim that for nullable vectors and scenarios when > we > > > turn off the non-nullable vector flag, the new changes do not introduce > > > performance regression. Such concern is plausible, as our changes add > some > > > if-else branches to the code, which may degrade performance. > > > > > > We give the benchmark results of the original code, as below. > > > > > > *(original ocde)* > > > > > > > > > > > > > > > > > > *Benchmark Mode Cnt Score Error > > > UnitsIntBenchmarks.getInt avgt 5 383.511 ± 0.156 > > > ns/opIntBenchmarks.isNull avgt 5 1274.271 ± 19.092 > > > ns/opIntBenchmarks.setIntDirectly avgt 5 15162.219 ± 194.956 > > > ns/opIntBenchmarks.setWithValueHolder avgt 5 15247.640 ± 153.103 > > > ns/opIntBenchmarks.setWithWriter avgt 5 28587.780 ± 160.458 > > > ns/op* > > > > > > By comparing this set of results with the above results with > non-nullable > > > vector flag disabled, little performance difference can be observed, > > > indicating no performance regression. > > > > > > By examining the generated assembly, it can be seen that JIT is smart > > > enough to remove the if-else branch completely (the below screen-shot > gives > > > an example where the if branch is optimized away in the assembly) > > > > > > So if some users do not like this feature, because their vectors are > > > always nullable, they can simply disable the flag, and no performance > > > difference can be observed. > > > > > > Would you please give some feedback? > > > If it looks good to you, maybe we can go ahead to support other types > of > > > vectors. > > > Thank you in advance. > > > > > > Liya Fan > > > > > > [image: image.png] > > > > > > [1] https://github.com/apache/arrow/pull/8147 > > > > > > On Fri, Mar 13, 2020 at 9:47 PM Fan Liya <liya.fa...@gmail.com> wrote: > > > > > >> Hi Jacques, > > >> > > >> Thanks a lot for your valuable comments. > > >> > > >> I agree with you that collapsing nullable and non-nullable > > >> implementations is a good idea, and it does not contradict with the > idea of > > >> introducing a fast code path, if it does not introduce much cost or > code > > >> complexity. > > >> > > >> The idea of word level checking is interesting. > > >> > > >> As you suggested, we will do more investigations and reconsider the > > >> problem from a broader perspective. > > >> > > >> Best, > > >> Liya Fan > > >> > > >> > > >> > > >> On Thu, Mar 12, 2020 at 9:27 AM Jacques Nadeau <jacq...@apache.org> > > >> wrote: > > >> > > >>> Generally Ive found that this isnt an important optimization in the > use > > >>> cases we see. Memory overhead, especially with our Java shared > allocation > > >>> scheme is nominal. Optimizing null checks at the word level usually > is > > >>> much > > >>> more impactful since non null and null runs are much more common on a > > >>> shorter window common than they seeing those declared. > > >>> > > >>> In other words, I'd suggest you look at your problem with a broader > > >>> perspective and see whether you're actually focused on optimizing the > > >>> most > > >>> important dimension. > > >>> > > >>> As an aside, the original Arrow Java code actually treated these > concepts > > >>> more distinctly and we consciously made a decision to collapse them > to > > >>> simplify real world use. > > >>> > > >>> I do think it makes to add a dirty read interface if you want. This > would > > >>> allow consumers of the interface to behave efficiently if they > wanted to. > > >>> > > >>> One last note, efficient evaluation and processing should generally > > >>> always > > >>> work at the validity word level. Adding an extra if check at the word > > >>> versus extra complexity of having an early out per batch seems like a > > >>> pretty small life in the grand scheme of processing. > > >>> > > >>> On Wed, Mar 11, 2020, 9:15 AM Brian Hulette <hulet...@gmail.com> > wrote: > > >>> > > >>> > > And there is a "nullable" metadata-only flag at the > > >>> > > Field level. Could the same kinds of optimizations be > implemented in > > >>> > > Java without introducing a "nullable" concept? > > >>> > > > >>> > Note Liya Fan did suggest pulling the nullable flag from the Field > > >>> when the > > >>> > vector is created in item (1) of the proposed changes. > > >>> > > > >>> > Brian > > >>> > > > >>> > On Wed, Mar 11, 2020 at 5:54 AM Fan Liya <liya.fa...@gmail.com> > wrote: > > >>> > > > >>> > > Hi Micah, > > >>> > > > > >>> > > Thanks a lot for your valuable comments. Please see my comments > > >>> inline. > > >>> > > > > >>> > > > I'm a little concerned that this will change assumptions for at > > >>> least > > >>> > > some > > >>> > > > of the clients using the library (some might always rely on the > > >>> > validity > > >>> > > > buffer being present). > > >>> > > > > >>> > > I can understand your concern and I am also concerned. > > >>> > > IMO, the client should not depend on this assumption, as the > > >>> > specification > > >>> > > says "Arrays having a 0 null count may choose to not allocate the > > >>> > validity > > >>> > > bitmap." [1] > > >>> > > That being said, I think it would be safe to provide a global > flag to > > >>> > > switch on/off the feature (as you suggested). > > >>> > > > > >>> > > > I think this is a good feature to have for the reasons you > > >>> mentioned. > > >>> > It > > >>> > > > seems like there would need to be some sort of configuration > bit > > >>> to set > > >>> > > for > > >>> > > > this behavior. > > >>> > > > > >>> > > Good suggestion. We should be able to switch on and off the > feature > > >>> with > > >>> > a > > >>> > > single global flag. > > >>> > > > > >>> > > > But, I'd be worried about code complexity this would > > >>> > > > introduce. > > >>> > > > > >>> > > I agree with you that code complexity is an important factor to > > >>> consider. > > >>> > > IMO, our proposal should not involve too much code change, or > > >>> increase > > >>> > code > > >>> > > complexity too much. > > >>> > > To prove this, maybe we need to show some small experimental code > > >>> change. > > >>> > > > > >>> > > Best, > > >>> > > Liya Fan > > >>> > > > > >>> > > [1] > https://arrow.apache.org/docs/format/Columnar.html#logical-types > > >>> > > > > >>> > > On Wed, Mar 11, 2020 at 1:53 PM Micah Kornfield < > > >>> emkornfi...@gmail.com> > > >>> > > wrote: > > >>> > > > > >>> > > > Hi Liya Fan, > > >>> > > > I'm a little concerned that this will change assumptions for at > > >>> least > > >>> > > some > > >>> > > > of the clients using the library (some might always rely on the > > >>> > validity > > >>> > > > buffer being present). > > >>> > > > > > >>> > > > I think this is a good feature to have for the reasons you > > >>> mentioned. > > >>> > It > > >>> > > > seems like there would need to be some sort of configuration > bit > > >>> to set > > >>> > > for > > >>> > > > this behavior. But, I'd be worried about code complexity this > would > > >>> > > > introduce. > > >>> > > > > > >>> > > > Thanks, > > >>> > > > Micah > > >>> > > > > > >>> > > > On Tue, Mar 10, 2020 at 6:42 AM Fan Liya <liya.fa...@gmail.com > > > > >>> wrote: > > >>> > > > > > >>> > > > > Hi Wes, > > >>> > > > > > > >>> > > > > Thanks a lot for your quick reply. > > >>> > > > > I think what you mentioned is almost exactly what we want to > do > > >>> in > > >>> > > > Java.The > > >>> > > > > concept is not important. > > >>> > > > > > > >>> > > > > Maybe there are only some minor differences: > > >>> > > > > 1. In C++, the null_count is mutable, while for Java, once a > > >>> vector > > >>> > is > > >>> > > > > constructed as non-nullable, its null count can only be 0. > > >>> > > > > 2. In C++, a non-nullable array's validity buffer is null, > while > > >>> in > > >>> > > Java, > > >>> > > > > the buffer is an empty buffer, and cannot be changed. > > >>> > > > > > > >>> > > > > Best, > > >>> > > > > Liya Fan > > >>> > > > > > > >>> > > > > On Tue, Mar 10, 2020 at 9:26 PM Wes McKinney < > > >>> wesmck...@gmail.com> > > >>> > > > wrote: > > >>> > > > > > > >>> > > > > > hi Liya, > > >>> > > > > > > > >>> > > > > > In C++ we elect certain faster code paths when the null > count > > >>> is 0 > > >>> > or > > >>> > > > > > computed to be zero. When the null count is 0, we do not > > >>> allocate a > > >>> > > > > > validity bitmap. And there is a "nullable" metadata-only > flag > > >>> at > > >>> > the > > >>> > > > > > Field level. Could the same kinds of optimizations be > > >>> implemented > > >>> > in > > >>> > > > > > Java without introducing a "nullable" concept? > > >>> > > > > > > > >>> > > > > > - Wes > > >>> > > > > > > > >>> > > > > > On Tue, Mar 10, 2020 at 8:13 AM Fan Liya < > liya.fa...@gmail.com > > >>> > > > >>> > > wrote: > > >>> > > > > > > > > >>> > > > > > > Dear all, > > >>> > > > > > > > > >>> > > > > > > A non-nullable vector is one that is guaranteed to > contain no > > >>> > > nulls. > > >>> > > > We > > >>> > > > > > > want to support non-nullable vectors in Java. > > >>> > > > > > > > > >>> > > > > > > *Motivations:* > > >>> > > > > > > 1. It is widely used in practice. For example, in a > database > > >>> > > engine, > > >>> > > > a > > >>> > > > > > > column can be declared as not null, so it cannot contain > null > > >>> > > values. > > >>> > > > > > > 2.Non-nullable vectors has significant performance > advantages > > >>> > > > compared > > >>> > > > > > with > > >>> > > > > > > their nullable conterparts, such as: > > >>> > > > > > > 1) the memory space of the validity buffer can be > saved. > > >>> > > > > > > 2) manipulation of the validity buffer can be bypassed > > >>> > > > > > > 3) some if-else branches can be replaced by sequential > > >>> > > instructions > > >>> > > > > (by > > >>> > > > > > > the JIT compiler), leading to high throughput for the CPU > > >>> > pipeline. > > >>> > > > > > > > > >>> > > > > > > *Potential Cost:* > > >>> > > > > > > For nullable vectors, there can be extra checks against > the > > >>> > > > > nullablility > > >>> > > > > > > flag. So we must change the code in a way that minimizes > the > > >>> > cost. > > >>> > > > > > > > > >>> > > > > > > *Proposed Changes:* > > >>> > > > > > > 1. There is no need to create new vector classes. We add > a > > >>> final > > >>> > > > > boolean > > >>> > > > > > to > > >>> > > > > > > the vector base classes as the nullability flag. The > value > > >>> of the > > >>> > > > flag > > >>> > > > > > can > > >>> > > > > > > be obtained from the field when creating the vector. > > >>> > > > > > > 2. Add a method "boolean isNullable()" to the root > interface > > >>> > > > > ValueVector. > > >>> > > > > > > 3. If a vector is non-nullable, its validity buffer > should > > >>> be an > > >>> > > > empty > > >>> > > > > > > buffer (not null, so much of the existing logic can be > left > > >>> > > > unchanged). > > >>> > > > > > > 4. For operations involving validity buffers (e.g. > isNull, > > >>> get, > > >>> > > set), > > >>> > > > > we > > >>> > > > > > > use the nullability flag to bypass manipulations to the > > >>> validity > > >>> > > > > buffer. > > >>> > > > > > > > > >>> > > > > > > Therefore, it should be possible to support the feature > with > > >>> > small > > >>> > > > code > > >>> > > > > > > changes. > > >>> > > > > > > > > >>> > > > > > > BTW, please note that similar behaviors have already been > > >>> > supported > > >>> > > > in > > >>> > > > > > C++. > > >>> > > > > > > > > >>> > > > > > > Would you please give your valueable feedback? > > >>> > > > > > > > > >>> > > > > > > Best, > > >>> > > > > > > Liya Fan > > >>> > > > > > > > >>> > > > > > > >>> > > > > > >>> > > > > >>> > > > >>> > > >> >
