Leveraging Spatial and Temporal Correlations in Sparsified Mean Estimation

1 Introduction . Estimating the mean of a set of vectors collected from a distributed system of nodes is a subroutine that is at the core of many distributed data science applications . For instance , a fusion server performing inference may seek to estimate the mean of environmental measurements made by geographically distributed sensor nodes . While inference may be a one-shot task , model training is often inherently iterative in nature . For instance , model training using a federated learning framework [ 19 , 35 ] is divided into communication rounds , where in each round , the central server has to estimate the mean of model updates or gradients sent by edge clients . Other examples of such iterative methods include principal component analysis ( PCA ) using the power iteration methods and K-Means . In most such distributed learning and optimization applications , the nodes sending the data are highly resource-constrained and bandwidth-limited edge devices . Hence , sending high-dimensional vectors can be prohibitively expensive and slow . A simple yet effective solution to reduce the communication 35th Conference on Neural Information Processing Systems ( NeurIPS 2021 ) . cost is to send a sparsified vector containing only a subset of the elements of the original vector [ 30 ] . The Rand-k sparsification scheme chooses a subset of k elements uniformly at random , while the Top-k scheme selects the k highest magnitude indices . Other more advanced schemes such as [ 34 , 36 ] optimize the weights assigned by a node to different elements of its vector when sparsifying it and sending it to the central server . A common thread in the existing techniques [ 36 , 34 , 21 ] is that they are agnostic to the fact that in many practical applications , the vectors sent by the nodes are correlated across different nodes and over consecutive rounds of iterative algorithms that use mean estimation as a subroutine . For example , in a sensor data fusion applications , sensor nodes in close promixity will make similar environmental measurements and their resulting data vectors will be correlated . In a federated learning application , model updates sent by a node tend to be similar across consecutive communication rounds . The focus of this work is to take into account these spatial and temporal correlations when combining sparsified vectors sent by nodes and estimating their mean . 1.1 Main Contributions . In this work , we carefully design the aggregation method used by the central server to estimate the mean of sparsified vectors received from different nodes . This approach does not introduce additional memory requirement or computation at the nodes . As a result , our proposed mean estimators are compatible with any sparsification method used by the nodes . For the purpose of this paper and the theoretical analysis of the estimation error , we consider random sparsification at the nodes and focus on unbiased mean estimation . However , other sparsification techniques including Top-k sparsification and the methods proposed in [ 34 , 36 ] can also be used by the nodes , albeit at the cost of additional local computation . Finally , while our goal is to leverage temporal and spatial correlations , we present methods that do not explicitly need information about the correlation ; the estimators only consider the fact that such correlation exists . By following this design philosophy of requiring little or no side information and additional local computation , this paper makes the following contributions : 1 . Leveraging Spatial Correlation : In random sparsification , a randomly chosen subset of k out of d elements of the vector is sent by a node to the central server . The standard approach is for the server to decode via simple averaging of the vectors assuming the missing values to be zero , and scaling with an appropriate constant to get an unbiased mean estimate . Instead , in Section 3 , we propose a family of estimators called Rand-k-Spatial that adjust the weights assigned to different vectors according to a parameter that represents the amount of spatial correlation . When the correlation parameter is not known , we propose an average-case estimator that works well in practice . 2 . Leveraging Temporal Correlation : If the vectors sent by a node are similar across iterations , previously sent elements of the vectors can be used to fill in the missing elements of a sparsified vector . In Section 4 , we build on this idea of leveraging these temporal correlations and propose the Rand-k-Temporal estimator . While our estimator requires additional memory at the central server , it does not add or change any computation or storage at the nodes , which are more likely to be resource-constrained . Along with designing these estimators and theoretically analyzing their mean squared error , in Section 5 we present experiments showing that the estimator error can be drastically reduced when there are spatial and temporal correlations . 1.2 Related Work . The problem of communication-efficient mean estimation when the data is distributed across several nodes is considered in [ 40 , 13 , 7 , 33 , 10 , 24 ] . Among them , [ 40 , 13 , 7 ] consider statistical mean estimation assuming i.i.d . data distribution across nodes . Works [ 33 , 10 , 24 , 17 ] consider empirical mean estimation without assuming any statistical distribution on the data , and quantize the vectors to a small number of bits . There is a significant recent interest in considering the communicationefficient ( empirical ) mean estimation problem in the context of distributed stochastic gradient descent , see e.g. , [ 1 , 2 , 12 , 5 , 37 , 36 , 34 , 20 , 4 , 23 , 6 , 14 , 27 ] . These works can broadly be partitioned into three categories : ( i ) Quantization : encoding each element of the vectors to a small number of bits [ 18 , 1 , 12 , 5 , 37 , 20 , 23 , 27 ] , ( ii ) Sparsification : sending only a subset of elements of the vectors [ 2 , 34 , 36 ] . ( iii ) Generic compression operators : defining a class of compressors ( with specific properties such as unbiasedness , bounded variance ) that encompass quantization and sparsification as special cases [ 20 , 14 ] . We focus our attention to sparsification since it is orthogonal to quantization and the two can also be combined , see e.g. , [ 4 ] . To the best of our knowledge , spatial correlation across nodes has not yet been considered in the context of quantized and sparsified mean estimation . Temporal correlation has been considered in some recent sparsification works [ 27 , 14 ] and errorfeedback [ 20 , 30 ] . However , these methods change the sparsification method at the nodes and require additional memory and local computation . In contrast , we only modify the server-side aggregation method . See Section 4 for more details of prior work on exploiting temporal correlation . 2 Problem Formulation . System Model and Notation . We consider a setup consisting of n geographically distributed nodes , and a central server . Each node generates a d−dimensional vector xi = [ xi1 , . . . , xid ] T for i = 1 , . . . , n , where xij denotes the jth element of node i ’ s vector xi . Note that we do not assume any specific distribution from which these vectors are generated . Spatial correlations between the vectors in Section 3 are measured using the sum of their inner products . In Section 4 we measure temporal correlations by considering the L2 distance between the vectors sent by a node in different iterations . The central server seeks to estimate the mean of these vectors : x̄ = 1 n n∑ i=1 xi ( 1 ) In order to save communication cost , each node sends a sparsified version hi of its data vector xi to the central server . The estimated mean x̂ is a function of the h1 , . . . , hn sent by the n nodes . Rand-k Sparsified Mean Estimation . Random sparsification [ 30 ] is a simple and commonly used way to sparsify a vector in order to save communication cost . In the Rand-k scheme , node i selects k elements of xi uniformly at random and sends these ( along with their position information ) to the central node . Thus , the jth element of the sparsified vector hi is hij = xij if node i sends coordinate j ( with probability k/d ) and is 0 otherwise . The central node computes the mean estimate as x̂ = 1 n d k n∑ i=1 hi . ( 2 ) The scaling factor dk ensures that d kE [ hi ] = xi and therefore E [ x̂ ] = x̄ , i.e. , the estimate is unbiased . For the purpose of this paper , we will focus on unbiased random sparsification at the nodes . However , our techniques for leveraging spatial and temporal correlations can be applied to biased sparsification methods such as Top-k as well . Estimation Error Metric . As done in most previous works , we measure the quality of the estimator in terms of the mean squared error ( MSE ) Ex̂ [ ‖x̂− x̄‖2 ] , where the expectation is taken over the choice of the indices that are retained by each of the nodes when sparsifying their data vectors xi , see , e.g. , [ 33 , 12 , 10 ] . The following result , known in folklore , quantifies error in estimating the true mean x̄ ( proof in Appendix A ) . Lemma 1 ( Rand-k Estimation Error ) . The mean squared error ( MSE ) of estimate x̂ produced by the Rand-k sparsification scheme described above is given by E [ ‖x̂− x̄‖2 ] = 1 n2 ( d k − 1 ) R1 ( 3 ) where R1 = ∑n i=1 ‖xi‖ 2 , the sum of the squared magnitudes of the data vectors . 3 Improving Random-k by Leveraging Spatial Correlations Consider Rand-k sparsification protocol ( described in Section 2 ) where node i sparsifies its vector xi to hi by retaining only k randomly chosen elements and setting the rest to 0 , and sends hi to the server . The sampled coordinates are scaled by a factor of d/k to get an unbiased estimate of each node vector . However , consider the case where each node has the same vector , i.e. , x1 = x2 = . . . = xn . If Mj > 0 nodes send their jth coordinate , then the jth coordinate of the mean can be exactly estimated as x̂j = ( ∑ i hij ) /Mj . On the other hand , the scaling factor of d/k in the Rand-k estimator can introduce a large error depending on the value of Mj ( e.g. , if Mj = 1 but d/k = 10 ) . The above example suggests that there are scenarios in which the Rand-k estimator , despite being unbiased , can incur a large MSE . In this section , we will introduce the Rand-k-Spatial family of unbiased mean estimators ( illustrated in Fig . 1 ) whose members compute mean estimates depending on the amount of correlation between the data at the nodes . We will show that the conventional Rand-k estimator is a member of this family and gives the lowest MSE ( among the family ) only when the data is uncorrelated , while other members of the family can yield lower values of MSE depending upon the level of correlation . We will also describe how to obtain the estimator from this family that minimizes the MSE when the correlation between the data is known and present an estimator that gives lower MSE than Rand-k on average when correlations are unknown . The Rand-k-Spatial family of estimators . Let Mj be the number of nodes that send their jth coordinate . Observe thatMj is a binomial random variable that takes values in the range { 0 , 1 , . . . , n } with Pr ( Mj = m ) = ( n m ) pm ( 1− p ) n−m for all j , where p = k/d . Instead of scaling each element by the constant d/k as in Rand-k ( cf . ( 2 ) ) , we propose the Rand-k-Spatial family of estimators wherein the scaling is computed as a function of Mj and the level of spatial correlation between the vectors . In particular , we propose the following estimate for the jth coordinate of the mean , x̂j = 1 n β̄ T ( Mj ) n∑ i=1 hij , ( 4 ) where β̄ , ( kdEMj |Mj≥1 [ 1 T ( Mj ) ] ) −1 . The function T ( Mj ) changes the scaling of hij depending on Mj , the number of nodes that send the jth coordinate . Lemma 2 ( Rand-k-Spatial estimator Unbiasedness ) . Following the definition of β̄ , we have that the Rand-k-Spatial estimator in ( 4 ) is unbiased , i.e , E [ x̂j ] = 1 n n∑ i=1 xij ( 5 ) Observe that the Rand-k-estimator of ( 2 ) can be obtained by setting T ( Mj ) = 1 ∀Mj ≥ 1 . The estimator described at the beginning of this Section for the case where all the xi vectors are identical can be obtained ( up to a constant ) by setting T ( Mj ) = Mj ∀Mj ≥ 1 ( the constant scaling by β̄ is required for unbiasedness ) . In general , the estimators obtained by the different settings of T ( Mj ) form the Rand-k-Spatial family of estimators . Our goal now is to find the best estimator from this family for the task of mean estimation . Towards this end , we first evaluate the mean estimation error for members of this family , and then show how the best choice of T ( Mj ) is a function of the amount of spatial correlation . Theorem 1 ( Rand-k-Spatial Estimation Error ) . The mean squared error of any member of the Rand-k-Spatial family is given by E [ ‖x̂− x̄‖2 ] = 1 n2 ( d k − 1 ) R1 + 1 n2 ( c1R1 − c2R2 ) ( 6 ) where R1 = ∑n i=1 ‖xi‖ 2 and R2 = 2 ∑n i ∑n j=i+1〈xi , xj〉 . The parameters c1 , c2 depend on the choice of T ( . ) as c1 = β̄ 2 n∑ m=1 k dT ( m ) 2 ( n− 1 m− 1 ) ( k d ) m−1 ( 1− k d ) n−m − d k ( 7 ) and c2 = 1− β̄2 n∑ m=2 k2 d2T ( m ) 2 ( n− 2 m− 2 ) ( k d ) m−2 ( 1− k d ) n−m . ( 8 ) Choosing the best estimator . Comparing ( 6 ) with ( 3 ) , we see that if the data x1 , . . . , xn and function T ( ) is such that c1R1 < c2R2 then the MSE of the corresponding estimator in the Rand-k-Spatial family is guaranteed to be lower than the Rand-k estimator of Section 2 . In general , since the MSE depends on the function T ( . ) through c1 and c2 , we can find the T ( . ) that minimizes the MSE as shown below . Theorem 2 ( Rand-k-Spatial minimum MSE ) . For any set of data vectors x1 , . . . , xn , the ratio R2R1 always lies in [ −1 , n − 1 ] , and the estimator within the Rand-k spatial family of estimators that minimizes the MSE in ( 6 ) , can be obtained by setting T ∗ ( m ) = 1 + R2 R1 m− 1 n− 1 , for m = 1 , . . . , n. ( 9 ) Thus , the best mean estimator from the Rand-k-family changes as the data ( and consequently R1 , R2 ) changes . We now discuss certain special cases of this . 1 . Rand-k ( Best when R2/R1 = 0 ) The conventional Rand-k estimator ( corresponding to T ∗ ( m ) = 1 , ∀m ) minimizes the MSE only in a special case which is , in fact , the case when R2 = 2 ∑n i ∑n j=i+1〈xi , xj〉 = 0 . A natural setting where this could occur is when the node vectors are orthogonal to each other , i.e. , 〈xi , xj〉 = 0 ∀i , j ∈ [ n ] . In such cases , it can be said that the conventional Rand-k estimator is , in fact , the best out of the Rand-k-Spatial family of estimators . However , when R2/R1 6= 0 , Rand-k is not the best estimator , and for each value of R2/R1 , a different estimator ( corresponding to a different value of T ∗ ( . ) ) minimizes the MSE . 2 . Rand-k-Spatial ( Max ) ( Best when R2/R1 = n− 1 ) Recall the example described at the beginning of this section where xi = xj ∀i , j ∈ [ n ] . In this case , R2/R1 = n− 1 , and T ∗ ( m ) = m , ∀m gives the optimal estimator . We will call this the Rand-k-Spatial ( Max ) estimator since it corresponds to the maximum value of R2/R1 . Thus , the optimal estimators are very different when the vectors are uncorrelated as opposed to when the vectors are highly correlated . 3 . Rand-k-Spatial ( Avg ) ( Best on Average ) At this point it is quite clear that no single value of T ( . ) will minimize MSE for all datasets . However , from ( 9 ) it is also evident that to derive the best T ( . ) we need to know the exact values of R2/R1 at the central server which does not seem possible without communicating the entire vector at each node ( particularly for computing R2 ) which we wish to avoid . Therefore , we propose the following choice of T ( . ) as a default setting , and subsequently show empirically that it does better than either of the above two extremes on average . T̃ ( m ) = 1 + n 2 m− 1 n− 1 ( 10 ) Comparing with ( 9 ) , we see that this is the best estimator when R2/R1 = n/2 which is the mid-point of the interval in which R2/R1 lies ( as per Theorem 2 ) . It can also be shown that this minimizes the expectation over MSE if R2/R1 is distributed uniformly over its range . We will now present simulations to show that this estimator gives close to the best MSE across the entire range of R2/R1 , and will show in Section 5 that it can outperform state-of-the-art baselines for real distributed mean estimation tasks . Simulations . To illustrate the discussion above , we performed mean estimation for n = 10 100−dimensional vectors using k/d = 0.1 . Since n = 10 , R2/R1 ∈ [ −1 , 9 ] ( Theorem 2 ) . We vary R2/R1 over the entire range by starting with 5 all-1 vectors and 5 all- ( −1 ) vectors ( R2/R1 = −1 ) and then changing the signs of the elements ( one at a time ) of the 5 all- ( −1 ) vectors , until they are all1 ( R2/R1 = 9 ) . For details , see Appendix B . We plot the MSE of the Rand-k , Rand-k-Spatial ( Max ) , and Rand-k-Spatial ( Avg ) estimators in Fig . 2a . We also add an estimator corresponding to the best T ( . ) as in ( 9 ) and call it Rand-k-Spatial ( Opt ) . While knowing R2/R1 at the central server is unrealistic in practice , it illustrates the best-case scenario . In Fig . 2a , we see that Rand-k-Spatial ( Avg ) is in general quite close to Rand-k-Spatial ( Opt ) , i.e. , it works well for all values of R2/R1 , whereas Rand-k and Rand-k-Spatial ( Max ) work well only in the special cases where R2/R1 = 0 and R2/R1 = 9 . To check if R2/R1 can indeed take large values for which Rand-k would be suboptimal , we consider distributed power iteration which is a common protocol that uses distributed mean estimation [ 33 , 11 ] . The objective of the central server here is to estimate the top eigenvector of a matrix whose rows are partitioned among nodes . To simulate this , we partition the FMNIST and CIFAR-10 datasets among 100 nodes , either in an IID or Non-IID fashion and plot R2/R1 for the vectors in the first round of distributed power iteration on this data . Fig . 2b shows that R2/R1 is significantly larger than 0 in these cases , thus indicating that the data is well correlated . The values of R2/R1 continue to remain large even in subsequent round ( see Appendix B ) , thus justifying the need for using estimators other than the conventional Rand-k at each round . In Section 5 , we show that the Rand-k-Spatial ( Avg ) estimator can outperform Rand-k and various other baselines for distributed power iteration . 4 Improving Random-k by Leveraging Temporal Correlations Mean estimation is an important subtask in several iterative algorithms such as power iteration and SGD . We propose a mean estimation technique that leverages temporal correlations in the vectors x ( t ) i obtained by node i , where t is the iteration index of the algorithm . In Rand-k sparsification , each node i samples k out of d coordinates of its vector x ( t ) i to create a sparsified vector h ( t ) i and sends it to the server . In standard Rand-k sparsified mean estimation , the d− k unsampled coordinates of x ( t ) i are treated as 0 by the server when computing the mean estimate ( see ( 2 ) ) . Instead , we propose to allow the server to substitute the zeros with values from its memory that better represent the unsampled coordinates . Filling-in Unsampled Coordinates of a Sparsified Vector . Let b ( t ) 1 , b ( t ) 2 . . .b ( t ) n be n vectors stored in the server memory whose elements serve as substitutes for unsampled coordinates of x ( t ) i . Since these b ( t ) i s are stored at the server , there is no added computation or storage costs at the nodes . These b ( t ) i s can correspond to predicted values of x ( t ) i based on the temporal correlation across iterations . A natural idea is to set b ( t ) i in iteration t to previously sampled coordinates , instead of just substituting zeros . Based on this intuition , we propose the following strategy for updating b ( t ) i at the end of iteration t. Let b ( 0 ) i = 0 for all i ∈ [ n ] . As illustrated in Fig . 3 , we update b ( t ) i for t ≥ 1 as follows : b ( t+1 ) ij , { b ( t ) ij if node i does not send x ( t ) ij x ( t ) ij if node i sends x ( t ) ij . ( 11 ) One can further refine this approach by enforcing staleness constraints on b ( t ) i or by setting b ( t ) i to a sliding window average of previously sampled coordinates . The Rand-k-Temporal estimator . We now propose the Rand-k-Temporal estimator that enables the server to get an unbiased estimate of the mean x̄ for any arbitrary stored vectors bi used to fill in the unsampled coordinates of xi . We omit the iteration index t here for the purpose of brevity and because this estimator can be used even in non-iterative settings where the server knows a predicted value bi of the vector xi . Given the sparsified vector hi received from node i , the server can fill in the unsampled coordinates and create the vector h′i whose j-th coordinate is defined as : h′ij , { bij if node i does not send xij bij + d k ( xij − bij ) if node i sends xij ( 12 ) By definition , h′ij is an unbiased estimate of xij , because E [ h′ij ] = ( 1− kd ) bij + kd ( bij + dk ( xij − bij ) ) = xij . Using these filled-in vectors h′i obtained from the sparsified vectors hi and the vectors bi stored in server memory , we propose that the server computes the following mean estimate x̂ = 1 n n∑ i=1 h′i . ( 13 ) We see that when bi = 0 for all i , then the Rand-k-Temporal estimator reduces to the Rand-k estimator . Since h′i is an unbiased estimate of xi , it follows that the proposed mean estimate x̂ is also unbiased , that is E [ x̂ ] = x . In the following theorem , we evaluate the estimation error of this Rand-k-Temporal estimator in terms of the bis . Theorem 3 ( Rand-k-Temporal Estimation Error ) . The mean squared error of the Rand-k-Temporal estimator is given by , E [ ‖x̂− x̄‖2 ] = 1 n2 ( d k − 1 ) n∑ i=1 ‖xi − bi‖2 . ( 14 ) By comparing the above result with the Rand-k estimation error given in Lemma 1 , we see that Rand-k-Temporal effectively reduces the dependence of the MSE on ‖xi‖2 to ‖xi − bi‖2 . Thus , the Rand-k-Temporal estimator can significantly reduce the estimation error of the Rand-k estimator when bis are close to xis , resulting in ‖xi − bi‖2 < ‖xi‖2 . Reducing Storage Cost at the Server . In the naive implementation of the proposed Rand-kTemporal estimator , we need O ( nd ) storage/memory at the server to store vector bi from each device i . In practice , this memory cost can be easily reduced at the expense of a slightly higher MSE . Note that our result in Theorem 3 holds for arbitrary bi and thus the bi do not have to be necessarily different for each device . A natural idea to reduce the memory requirement at the server to just O ( d ) is to set b ( t+1 ) i = x̂ ( t ) for all i ∈ [ n ] , where x̂ ( t ) is the previous mean estimate . Further details and experimental results confirming the effectiveness of this strategy are shown in Appendix D. A Quadratic-Objective Case Study . Even with the simple choice of bi given in ( 11 ) , Rand-kTemporal estimation gives an order-wise improvement in the error over standard Rand-k estimation . We demonstrate this via the following case study of performing gradient descent on a quadratic objective . Consider that each node i has a quadratic local objective function given by Fi ( w ) = 1 2 ‖w − ei‖ 2 , where ei ∈ Rd is the local minimum of node i . The local minima are different across nodes due to heterogeneity of their objectives . The goal of the server is to minimize the global objective F ( w ) given by , F ( w ) = 1 n n∑ i=1 Fi ( w ) = 1 2n n∑ i=1 ‖w − ei‖2 , which is minimized by w∗ = 1 n n∑ i=1 ei . ( 15 ) Our simulation result in Fig . 4b shows how the mean estimation error for these node gradients changes as training progresses . For schemes that do not utilize temporal information , we see a non-zero estimation error due to the unsampled coordinates at all steps . Rand-k-Temporal , on the other hand , effectively allows the mean estimation error of the node vectors to converge to zero , by utilizing previous and current information on all coordinates while decoding . A consequence of this , as we show in the theorem below , is that Rand-k-Temporal converges to the true optima w∗ for a sufficiently small learning rate η. Theorem 4 . For our objective defined as in ( 15 ) , Gradient Descent using the Rand-k-Temporal estimator with η ≤ min { 1 ( 1+ 8n ( d k−1 ) ) , k2d } converges to the true optima w∗ as follows , E [ ∥∥∥w ( t+1 ) −w∗∥∥∥2 ] ≤ ( 1− η ) t+1 [ ∥∥w0 −w∗∥∥2 + 1 n n∑ i=1 ‖∇Fi ( w∗ ) ‖2 ] ( 16 ) where the expectation is taken over the randomness in the sparsification protocol across all steps . Comparison with Error-Feedback and Gradient Difference methods : The notion of errorfeedback [ 28 , 31 ] is considered for compressed gradient methods in [ 29 , 30 , 20 , 6 ] . In error-feedback , each node locally stores the difference between the actual and the compressed vector at each iteration ( known as error ) , and adds it to the vector generated at the next iteration ( feedback ) . For Rand-k sparsification , error-feedback corresponds to locally storing the unsampled coordinates for each iteration , and adding them back at the next iteration . Note that , with error-feedback , the server still utilizes the information on only k coordinates from each client , albeit with accumulated history . On the other hand , Rand-k-Temporal allows the server to leverage temporal correlations by utilizing historical data to substitute for unsampled coordinates . Another line of work [ 26 , 15 , 22 ] proposes that nodes quantize the difference of their current gradient and a local state to reduce the variance caused by quantization . Note that similar to error-feedback , these works require nodes to maintain a local state and perform extra computation to update the local state . This can incur significant overhead for resource-constrained nodes in many applications such as SGD for deep learning models with millions of parameters . Comparison with rTop-k and Induced Compressor : In practice , biased compressors such as Topk are observed to perform better due to their low empirical variance , however error-feedback is the only known mechanism to ensure convergence [ 30 , 20 , 6 , 14 ] . rTop-k [ 3 ] proposes to first find the top r magnitude coordinates of a vector and then randomly sample k out of these r coordinates . Note that rTop-k is still a biased compressor and is usually implemented with error feedback . The work of [ 14 ] proposes a general mechanism called induced compressor for constructing an unbiased compression operator from any biased compressor such as Top-k . The induced compressor first computes the error between the actual and compressed vector obtained using a biased compressor , compresses the error using an unbiased compressor , and sends it along with the compressed vector . Unlike Rand-k-Temporal , the induced compressor only operates on the vectors for the current iteration , and does not leverage temporal correlations . Comparison with Lazily Aggregated Gradient ( LAG ) Methods : LAG methods [ 8 , 32 , 39 , 9 ] seek to utilize temporal correlation to reduce communication . In particular , LAG adaptively skips node-toserver communication by identifying slowly varying gradients , thereby reusing old gradients over time . Unlike Rand-k-Temporal , LAG is not a sparsification scheme and incurs an extra computation cost to determine whether or not a node sends their vector to the server .

Summary: The paper proposes two new sparsification methods for the distributed mean estimation problem. One of the proposed strategies, Rand-k-Spatial, uses the spatial correlation of different nodes to have an unbiased estimator. The other strategy, Rand-k-Temporal, uses the temporal correlation of local vectors at consecutive iterations to fill the sparsified elements without an extra computation or storage cost at the resource-constrained nodes. The authors show that both methods outperform Random-k and Top-k sparsification methods empirically.