[Enhancement] Optimize the de-serialization of vector when reading from Doc Values

[navneet1v]

Description

In reference to the experiments done in here: #1049, I was found out that when we flip to exact search and use the doc values do the exact search we spend like 60-70% of the time in de-serialization of vectors from binary format to float array. Hence as part of that there was few solutions provided, capturing them as part of this issue. Experiment details:

Experiment 1 (Baseline to Find latency for doing distance calculation on the different size dataset with different dimensions)

Below are the steps that are performed to run this experiment:

1. For every dimension and every run, generate a dataset with required number of vectors in it. We choose 1k, 2k, 5k, 8k, 10k, 15k vectors count.
2. After query vector is generated.
3. Now distance calculation is done on the dataset for every run and every dimension. The time to do distance calculation is stored and this is what we see in the tables below. Lucene distance calculation functions are used to do the distance calculation. code ref
4. The experiment then collects all the latencies and format them in a table.

dimensions	1000_latency(ms)	2000_latency(ms)	5000_latency(ms)	8000_latency(ms)	10000_latency(ms)	15000_latency(ms)
64	0.0751	0.07755	0.18681	0.33033	0.40572	0.60473
128	0.11913	0.14973	0.38251	0.64307	0.92643	1.17756
256	0.21269	0.30679	0.78219	1.26894	1.8096	2.38399
512	0.38937	0.5992	1.50937	2.5473	3.08621	4.54336
768	0.60041	0.88308	2.35232	3.64441	4.63974	7.03523
968	0.5958	1.11113	2.90708	4.61883	5.92536	8.58447
1024	0.60106	1.20861	3.12053	4.83357	6.24293	9.18919
2048	1.16614	2.45611	6.04847	9.83145	12.46421	18.96537
4096	2.39995	4.86545	11.93488	19.87645	25.46671	40.70765

Experiment 2(Doing de-serialization on vectors and then doing distance calculation)

Below are the steps that are performed to run this experiment:

1. For every dimension and every run, generate a dataset with required number of vectors in it. We choose 1k, 2k, 5k, 8k, 10k, 15k vectors count. The vectors in this dataset are serialized using the same code we use in K-NN(Code ref).
2. After query vector is generated. We don’t need to serialize the query as this is not what happens in the exact search.
3. Now to do the distance calculation we first de-serialized the vectors in the dataset using same code used by k-NN(Code ref) and then performed the distance calculation using Lucene provided functions. This is done for different dimensions and dataset. The time taken in this full step de-serialization and distance calculation is what being provided in below table. Lucene distance calculation functions are used to do the distance calculation. code ref
4. The experiment then collects all the latencies and format them in a table.

So, the query latency include:

1. Time taken to de-serialize the vectors to floats.
2. Distance calculation between the query vector and indexed vector.

dimensions	1000_latency_serialized(ms)	2000_latency_serialized(ms)	5000_latency_serialized(ms)	8000_latency_serialized(ms)	10000_latency_serialized(ms)	15000_latency_serialized(ms)
64	0.5361	0.30428	0.77805	1.2804	1.8098	2.36014
128	0.43123	0.54735	1.24873	1.9306	2.48106	3.5845
256	0.58271	1.06349	2.07025	3.69847	4.90092	8.71385
512	0.79718	1.65931	4.18262	6.67985	8.53127	15.2144
768	1.15486	2.20225	5.68341	9.35401	12.03496	23.44284
968	1.4213	2.85394	7.74272	14.27054	14.97346	33.13775
1024	1.46418	3.15568	7.50078	14.56883	17.00156	29.74787
2048	2.97711	6.85896	16.52853	24.79757	30.17373	78.9689
4096	5.90317	11.74842	31.05969	64.13292	83.25422	122.25665

Combining the Results of Experiment 1 and Experiment 2

dimensions	1000_latency(ms)	1000_latency_serialized(ms)	2000_latency(ms)	2000_latency_serialized(ms)	5000_latency(ms)	5000_latency_serialized(ms)	8000_latency(ms)	8000_latency_serialized(ms)	10000_latency(ms)	10000_latency_serialized(ms)	15000_latency(ms)	15000_latency_serialized(ms)
64	0.0751	0.5361	0.07755	0.30428	0.18681	0.77805	0.33033	1.2804	0.40572	1.8098	0.60473	2.36014
128	0.11913	0.43123	0.14973	0.54735	0.38251	1.24873	0.64307	1.9306	0.92643	2.48106	1.17756	3.5845
256	0.21269	0.58271	0.30679	1.06349	0.78219	2.07025	1.26894	3.69847	1.8096	4.90092	2.38399	8.71385
512	0.38937	0.79718	0.5992	1.65931	1.50937	4.18262	2.5473	6.67985	3.08621	8.53127	4.54336	15.2144
768	0.60041	1.15486	0.88308	2.20225	2.35232	5.68341	3.64441	9.35401	4.63974	12.03496	7.03523	23.44284
968	0.5958	1.4213	1.11113	2.85394	2.90708	7.74272	4.61883	14.27054	5.92536	14.97346	8.58447	33.13775
1024	0.60106	1.46418	1.20861	3.15568	3.12053	7.50078	4.83357	14.56883	6.24293	17.00156	9.18919	29.74787
2048	1.16614	2.97711	2.45611	6.85896	6.04847	16.52853	9.83145	24.79757	12.46421	30.17373	18.96537	78.9689
4096	2.39995	5.90317	4.86545	11.74842	11.93488	31.05969	19.87645	64.13292	25.46671	83.25422	40.70765	122.25665

Time Spent in De-Serialization

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dimensions	1000_latency_diff(ms)	2000_latency_diff(ms)	5000_latency_diff(ms)	8000_latency_diff(ms)	10000_latency_diff(ms)	15000_latency_diff(ms)
64	0.461	0.22673	0.59125	0.95007	1.40408	1.75541
128	0.3121	0.39762	0.86622	1.28753	1.55463	2.40694
256	0.37002	0.7567	1.28807	2.42953	3.09132	6.32987
512	0.40781	1.06011	2.67325	4.13255	5.44505	10.67103
768	0.55445	1.31917	3.33109	5.7096	7.39523	16.40761
968	0.8255	1.7428	4.83564	9.65171	9.04811	24.55328
1024	0.86312	1.94707	4.38025	9.73526	10.75863	20.55868
2048	1.81097	4.40285	10.48007	14.96612	17.70952	60.00352
4096	3.50322	6.88298	19.12481	44.25647	57.78751	81.549

Experiments Observations

1. De-serialization is most expensive operation while doing the Search. We can see that with 512 dimensions and 5K vectors we are taking around 2.6 ms for de-serialization which is around 63% of the time(4.18ms taken to compute the distances) and around 69% for lower dimensions like 128.

Time Spent In De-Serialization in %age

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dimensions	1000_latency_diff	2000_latency_diff	5000_latency_diff	8000_latency_diff	10000_latency_diff	15000_latency_diff
64	85.99	74.51	75.99	74.20	77.58	74.38
128	72.38	72.64	69.37	66.69	62.66	67.15
256	63.50	71.15	62.22	65.69	63.08	72.64
512	51.16	63.89	63.91	61.87	63.82	70.14
768	48.01	59.90	58.61	61.04	61.45	69.99
968	58.08	61.07	62.45	67.63	60.43	74.09
1024	58.95	61.70	58.40	66.82	63.28	69.11
2048	60.83	64.19	63.41	60.35	58.69	75.98
4096	59.34	58.59	61.57	69.01	69.41	66.70

Experiment Conclusion

1. The above experiments showed us that its the De-Serialization that is taking up most of the time during the exact search.

Possible solutions:

1. Short Term: To improve the exact search time, we should enable a caching layer for this de-serialization so that we can improve the latencies for exact search. Along with this SIMD instructions set will help reducing the overall time.
2. Figure out better format to store the vectors. Like can we use .vec file which is used by lucene engine too.

Benefits

This will feature will not only boost the filtered search but will also help in reducing the latency for script score query for k-NN where we read these vectors to perform the exact search.

This will also provide us some gains during the merge operation of segments as we read the binary doc values there too.

[jmazanec15]

Caching could be potentially tricky. I wonder if we should first evaluate lucene's .vec approach and potentially move towards that instead of first implementing caching. In the long run, I think we should convert our codec to KnnVectorFormat anyways.

[navneet1v]

@jmazanec15 Yes caching can be tricky. I added some ideas which comes in my mind while thinking about how to reduce the latency. We can very well ignore and move to a better solution.

[vamshin]

Agree we should move to KnnVectorFormat in long term and piggy back on the improvements from Lucene. Should we create dedicated GitHub issue to merge the codec to Vector classes used by Lucene?

[navneet1v]

@vamshin we can do that. But we need to make sure that we are not loosing focus on what we want to achieve. Plus BWC can be challenge here.

[vamshin]

@navneet1v I think merging to KnnVectorFormat is one of the solution to get optimizations. If we think its a long term solution, I dont see having dedicated GitHub issue is a problem. I am fine if we can track it part of the same issue as well.