Online Algorithms (OA)111Some literatures denote OA by ‘smoothed online convex optimization’. (Chen et al., 2018, 2016; Renault and Rosén, 2012) and Online Convex Optimization (OCO) (Bubeck, 2011; Hazan, 2016; Shalev-Shwartz, 2012) are two important settings of online decision making. Methods in both OA and OCO settings are designed to make a decision at every round, and then use the decision as a response to the environment. Their major difference is outlined as follows.
For every round, methods in the setting of OA are able to know a loss function first, and then play a decision as the response to the environment.
However, for every round, methods in the setting of OCO have to play a decision before knowing the loss function. Thus, the environment may be adversarial to decisions of those methods.
Both of them have a large number of practical scenarios. For example, both the -server problem (Lee, 2018; Bansal et al., 2010) and the Metrical Task Systems (MTS) problem (Abernethy et al., 2010; Bubeck et al., 2019; Bansal et al., 2010) are usually studied in the setting of OA. Other problems include online learning (Sun et al., 2016; Yang et al., 2013; Wang et al., 2019; Li et al., 2018), online recommendation (Wang et al., 2016), online classification (Crammer et al., 2004; Bernstein et al., 2010), online portfolio selection (Li et al., 2013), and model predictive control (Morari and Lee, 1999) are usually studied in the setting of OCO.
Many recent researches begin to investigate performance of online methods in both OA and OCO settings by using dynamic regret with switching cost (Chen et al., 2018; Li et al., 2018). It measures the difference between the cost yielded by real-time decisions and the cost yielded by the optimal decisions. Comparing with the classic static regret (Bubeck, 2011), it has two major differences.
First, it allows optimal decisions to change within a threshold over time, which is necessary in the dynamic environment222Generally, the dynamic environment means the distribution of the data stream may change over time..
Second, the cost yielded by a decision consists of two parts: the operating cost and the switching cost, while the classic static regret only contains the operating cost.
The switching cost measures the difference between two successive decisions, which is needed in many practical scenarios such as service management in electric power network (Mookherjee et al., 2008), dynamic resource management in data centers (Lin et al., 2011; Lu et al., 2013; Wang et al., 2014). However, we still have little knowledge about the relation between the dynamic regret and the switching cost. In the paper, we are motivated by the following fundamental questions.
Does the switching cost impact the dynamic regret of an online method?
Does the problem of online decision making become more difficult due to the switching cost?
To answer those challenging questions, we investigate online mirror descent in settings of OA and OCO, and provide a new theoretical analysis framework. According to our analysis, we find an interesting observation, that is, the switching cost does impact on the dynamic regret in the setting of OA. But, it has no impact on the dynamic regret in the setting of OCO. Specifically, when the switching cost is measured by with , the dynamic regret for an OA method is where is the maximal number of rounds, and is the given budget of dynamics. But, the dynamic regret for an OCO method is , which is same with the case of no switching cost (György and Szepesvári, 2016; Zhao et al., 2018; Zinkevich, 2003; Hall and Willett, 2013). Furthermore, we provide a lower bound of dynamic regret, namely for the OCO setting. Since the lower bound is still same with the case of no switching cost (Zhao et al., 2018), it implies that the switching cost does not change the difficulty of the online decision making problem for the OCO setting. Comparing with previous results, our new analysis is more general than previous results. We define a new dynamic regret with a generalized switching cost, and provide new regret bounds. It is novel to analyze and provide the tight regret bound in the dynamic environment, since previous analysis cannot work directly for the generalized dynamic regert. In a nutshell, our main contributions are summarized as follows.
We propose a new general formulation of the dynamic regret with switching cost, and then develop a new analysis framework based on it.
We provide regret with for the setting of OA and regret for the setting of OCO by using the online mirror descent.
We provide a lower bound regret for the setting of OCO, which matches with the upper bound.
The paper is organized as follows. Section 2 reviews related literatures. Section 3 presents the preliminaries. Section 4 presents our new formulation of the dynamic regret with switching cost. Section 5 presents a new analysis framework and main results. Section 6 presents extensive empirical studies. Section 7 conludes the paper, and presents the future work.
2. Related work
In the section, we review related literatures briefly.
2.1. Competitive ratio and regret
Although the competitive ratio is usually used to analyze OA methods, and the regret is used to analyze OCO methods, recent researches aim to developing unified frameworks to analyze the performance of an online method in both settings (Blum and Burch, 2000; Abernethy et al., 2010; Antoniadis et al., 2018; Buchbinder et al., 2012; Bubeck et al., 2018; Andrew et al., 2013; Chen et al., 2015). (Blum and Burch, 2000) provides an analysis framework, which is able to achieve sublinear regret for OA methods and constant competitive ratio for OCO methods. (Abernethy et al., 2010; Buchbinder et al., 2012; Bubeck et al., 2018) uses a general OCO method, namely online mirror descent in the OA setting, and improves the existing competitive ratio analysis for -server and MTS problems. Different from them, we extend the existing regret analysis framework to handle a general switching cost, and focus on investigating the relation between regret and switching cost. (Antoniadis et al., 2018) provides a lower bound for the OCO problem in the competitive ratio analysis framework, but we provide the lower bound in the regret analysis framework. (Andrew et al., 2013; Chen et al., 2015) study the regret with switching cost in the OA setting, but the relation between them is not studied. Comparing with (Andrew et al., 2013; Chen et al., 2015), we extend their analysis, and present a more generalized bound of dynamic regret (see Theorem 1).
2.2. Dynamic regret and switching cost
Regret is widely used as a metric to measure the performance of OCO methods. When the environment is static, e.g., the distribution of data stream does not change over time, online mirror descent yields regret for convex functions and regret for strongly convex functions (Bubeck, 2011; Hazan, 2016; Shalev-Shwartz, 2012). When the distribution of data stream changes over time, online mirror descent yields regret for convex functions (György and Szepesvári, 2016), where is the given budget of dynamics. Additionally, (Zinkevich, 2003) first investigates online gradient descent in the dynamic environment, and obtains regret (by setting ) for convex . Note that the dynamic regret used in (Zinkevich, 2003) does not contain swtiching cost. (Hall and Willett, 2013, 2015) use similar but more general definitions of dynamic regret, and still achieves regret. Furthermore, (Zhao et al., 2018) presents that the lower bound of the dynamic regret is . Many other previous researches investigate the regret under different definitions of dynamics such as parameter variation (Mokhtari et al., 2016; Yang et al., 2016; Gao et al., 2018; Zhang et al., 2017a), functional variation (Jenatton et al., 2016; Besbes et al., 2015; Zhang et al., 2018b), gradient variation (Chiang et al., 2012), and the mixed regularity (Jadbabaie et al., 2015; Chen et al., 2017). Note that the dynamic regret in those previous studies does not contain switching cost, which is significantly different from our work. Our new analysis shows that this bound is achieved and optimal when there is switching cost in the regret (see Theorems 2 and 3). The proposed analysis framework thus shows how the switching cost impacts the dynamic regret for settings of OA and OCO, which leads to new insights to understand online decision making problems.
|Algo.||Make decision first?||Observe first?||Metric||Has SC?|
In the section, we present the preliminaries of online algorithms and online convex optimization, and highlight their difference. Then, we present the dynamic regret with switching cost, which is used to measure the performance of both OA methods and OCO methods.
3.1. Online algorithms and online convex optimization
OA assumes that the loss function, e.g., , is known before making the decision at every round. But, OCO assumes that the loss function, e.g., , is given after making the decision at every round.
The performance of an OA method is measured by using the competitive ratio (Chen et al., 2018), which is defined by
Here, is denoted by
where . is the given budget of dynamics. It is the best offline strategy, which is yielded by knowing all the requests beforehand (Chen et al., 2018). Note that is the switching cost yielded by at the -th round. But, OCO is usually measured by the regret, which is defined by
where . is also the given budget of dynamics. Note that the regret in classic OCO algorithm does not contain the switching cost.
To make it clear, we use Table 1 to highlight their differences.
3.2. Dynamic regret with switching cost
Although the analysis framework of OA and OCO is different, the dynamic regret with switching cost is a popular metric to measure the performance of both OA and OCO (Chen et al., 2018; Li et al., 2018). Formally, for an algorithm , its dynamic regret with switching cost is defined by
where . Here, represents the switching cost at the -th round. is the given budget of dynamics in the dynamic environment. When , all optimal decisions are same. With the increase of , the optimal decisions are allowed to change to follow the dynamics in the environment. It is necessary when the distribution of data stream changes over time.
3.3. Notations and Assumptions.
We use the following notations in the paper.
The bold lower-case letters, e.g.,
, represent vectors. The normal letters, e.g.,, represent a scalar number.
represents a general norm of a vector.
represents Cartesian product, namely, . has the similar meaning.
Bregman divergence is defined by .
represents a set of all possible online methods, and represents some a specific online method.
represents ‘less than equal up to a constant factor’.
represents the mathematical expectation operator.
Assumption 1 ().
The following basic assumptions are used throughout the paper.
For any , we assume that is convex, and has -Lipschitz gradient.
The function is -strongly convex, that is, for any and , .
For any and , there exists a positive constant such that
For any , there exists a positive constant such that
4. Dynamic regret with generalized switching cost
In the section, we propose a new formulation of dynamic regret, which contains a generalized switching cost. Then, we highlight the novelty of this formulation, and present the online mirror decent method for setting of OA and OCO.
For an algorithm , it yields a cost at the end of every round, which consists of two parts: operating cost and switching cost. At the -th round, the operating cost is incurred by , and the switching cost is incurred by with . The optimal decisions are denoted by , which is denoted by
Here, is denoted by
is a given budget of dynamics, which measures how much the optimal decision, i.e., can change over . With the increase of , those optimal decisions can change over time to follow the dynamics in the environment effectively.
Denote an optimal method , which yields the optimal sequence of decisions . Its total cost is denoted by
Similarly, the total cost of an algorithm is denoted by
Definition 1 ().
For any algorithm , its dynamic regret with switching cost is defined by
Our new formulation of the dynamic regret makes a balance between the operating cost and the switching cost, which is different from the previous definition of the dynamic regret in (Zinkevich, 2003; György and Szepesvári, 2016; Hall and Willett, 2013).
Note that the freedom of with allows our new dynamic regret to measure the performance of online methods for a large number of problems. Some problems such as dynamic control of data centers (Lin et al., 2012), stock portfolio management (Li and Hoi, 2014), require to be sensitive to the small change between successive decisions, and the switching cost in these problems is usually bounded by . But, many problems such as dynamic placement of cloud service (Zhang et al., 2012) need to bound the large change between successive decisions effectively, and the switching cost in these problems is usually bounded by .
4.2. Novelty of the new formulation
Support more general switching cost. (Chen et al., 2018) defines the dynamic regret with switching cost by (1). It is a special case of our new formulation (2) by setting . The sequence of optimal decisions is dominated by and , and does not change over . is thus impacted by for the given and . Generally, is more sensitive to measure the slight change between and than . But, for some problems such as the dynamic placement of cloud service (Zhang et al., 2012), the switching cost at the -th round is usually measured by , instead of . The previous formulation in (Chen et al., 2018) is not suitable to bound the switching cost for those problems. Benefiting from , (2) supports more general switching cost than previous work.
Support more general convex . (Li et al., 2018) defines the the dynamic regret with switching cost by
and they use to bound the regret. Here, . It implicitly assumes that the difference between and are bounded. It is reasonable for a strongly convex function , but may not be guaranteed for a general convex function . Additionally, (Li et al., 2018) uses to bound the switching cost, which is more sensitive to the significant change than . But, it is less effective to bound the slight change between them, which is not suitable for many problems such as dynamic control of data centers (Lin et al., 2012).
We use mirror descent (Beck and Teboulle, 2003) in the online setting, and present the algorithm MD-OA for the OA setting and the algorithm MD-OCO for the OCO setting, respectively.
As illustrated in Algorithms 1 and 2, both MD-OA and MD-OCO are performed iteratively. For every round, MD-OA first observes the loss function , and then makes the decision at the -th round. But, MD-OCO first makes the decision , and then observe the loss function . Therefore, MD-OA usually makes the decision based on the observed for the current round, but MD-OCO has to predict a decision for the next round based on the received .
Note that both MD-OA and MD-OCO requires to solve a convex optimizaiton problem to update . The complexity is dominated by the domain and the distance function . Besides, both of them lead to memory cost. They lead to comparable cost of computation and memory.
5. Theoretical analysis
In this section, we present our main analysis results about the proposed dynamic regret for both MD-OA and MD-OCO, and discuss the difference between them.
5.1. New bounds for dynamic regret with switching cost
The upper bound of dynamic regret for MD-OA is presented as follows.
Theorem 1 ().
Remark 1 ().
When , MD-OA yields dynamic regret, which achieves the state-of-the-art result in (Chen et al., 2018). When , MD-OA yields dynamic regret, which is a new result as far as we know.
However, we find different result for MD-OCO. The switching cost does not have an impact on the dynamic regret.
Theorem 2 ().
Remark 2 ().
MD-OCO still yields dynamic regret (György and Szepesvári, 2016) when there is no switching cost. It shows that the switching cost does not have an impact on the dynamic regret.
Before presenting the discussion, we show that MD-OCO is the optimum for dynamic regret because the lower bound of the problem matches with the upper bound yielded by MD-OCO.
Theorem 3 ().
Under Assumption 1, the lower bound of the dynamic regret for the OCO problem is
Switching cost has a significant impact on the dynamic regret for the setting of OA. According to Theorem 1, the switching cost has a significant impact on the dynamic regret of MD-OA. Given a constant , a small leads to a strong dependence on , and a large leads to a weak dependence on . The reason is that a large leads to a large learning rate, which is more effective to follow the dynamics in the environment than a small learning rate.
Switching cost does not have an impact on the dynamic regret for the setting of OCO. According to Theorem 2 and Theorem 3, the dynamic regret yielded by MD-OCO is tight, and MD-OCO is the optimum for the problem. Although the switching cost exists, the dynamic regret yielded by MD-OCO does not have any difference.
As we can see, there is a significant difference between the OA setting and the OCO setting. The reasons are presented as follows.
MD-OA makes decisions after observing the loss function. It has known the potential operating cost and switching cost for any decision. Thus, it can make decisions to achieve a good tradeoff between the operating cost and switching cost.
MD-OCO make decisions before observing the loss function. It only knows the historical information and the potential switching cost, and does not know the potential operating cost for any decision at the current round. In the worst case, if the environment provides an adversary loss function to maximize the operating cost based on the decision played by MD-OCO, MD-OCO has to lead to regret even for the case of no switching cost (György and Szepesvári, 2016). Although the potential switching cost is known, MD-OCO cannot make a better decision to reduce the regret due to unknown operating cost.
6. Empirical studies
In this section, we evaluate the total regret and the regret caused by switching cost for settings of both OA and OCO by running online mirror decent. Our experiments show the importance of knowing loss function before making a decision.
6.1. Experimental settings
We conduct binary classification by using the logistic regression model. Given an instanceand its label , the loss function is
In experiments, we let .
We test four methods, including MD-OA, i.e., Algorithm 1, and MD-OCO, i.e., Algorithm 2, online balanced descent (Chen et al., 2018) denoted by BD-OA in the experiment, and multiple online gradient descent (Zhang et al., 2017b) denoted by MGD-OCO in the experiment. Both MD-OA and BD-OA are two variants of online algorithm, and similarily both MD-OCO and MGD-OCO are two variants of online convex optimization. We test those methods on three real datasets: usenet1333http://lpis.csd.auth.gr/mlkd/usenet1.rar, usenet2444http://lpis.csd.auth.gr/mlkd/usenet2.rar, and spam555http://lpis.csd.auth.gr/mlkd/concept_drift/spam_data.rar. The distributions of data streams change over time for those datasets, which is just the dynamic environment as we have discussed. More details about those datasets and its dynamics are presented at: http://mlkd.csd.auth.gr/concept_drift.html.
We use the average loss to test the regret, because they have the same optimal reference points . For the -th round, the average loss is defined by
where is the instance at the -th round, and is its label. Besides, we evaluate the average loss caused by operating cost separately, and denote it by OL. Similarly, SL represents the average loss caused by switching cost.
In experiment, we set . Since , , and
are usually not known in practical scenarios, the learning rate is set by the following heuristic rules. We choose the learning ratefor the -th iteration, where is a given constants by the following rules. First, we set a large value . Then, we iteratively adjust the value of by when cannot let the average loss converge. If the first appropriate can let the average loss converge, it is finally chosen as the optimal learning rate. We use the similar heuristic method to determine other parameters, e.g., the number of inner iterations in MGD-OCO. Finally, the mirror map function is for BD-OA.
6.2. Numerical results
As shown in Figure 1, both MD-OA and BD-OA are much more effcetive than MD-OCO and MGD-OCO to decrease the average loss during a few rounds of begining. Those OA methods yield much smaller average loss than OCO methods. The reason is that OA knows the loss function before making decision . But, OCO has to make decision before know the loss function. Benefiting from knowing the loss function , OA reduces the average loss more efffectively than OCO. It matches with our theoretical analysis. That is, Algorithm 1 leads to regret, but Algorithm 2 leads to regret. When , OA tends to lead to smaller regret than OCO. The reason is that OA knows the potential loss before playing a decision for every round. But, OCO works in an adversary environment, and it has to play a decision before knowing the potential loss. Thus, OA is able to play a better decision than OCO to decrease the loss. Additionally, we observe that both MD-OA and BD-OA reduce much more average loss than MD-OCO and MGD-OCO for a large , which validates our theoretical results nicely. It means that OA is more effective to reduce the switching cost than OCO for a large . Specifically, as shown in Figure 2, the average loss caused by switching cost of OA methods, i.e., MD-OA(SL), has unsignificant changes, but that of OCO methods, i.e., MD-OCO(SL), has remarkable increase for a large .
When handling the whole dataset, the final difference of switching cost between MD-OA and MD-OCO is shown in Figure 3. Here, the difference of switching cost is measured by using average loss caused by switching cost of MD-OCO minus corresponding average loss caused by switching cost of MD-OA. As we can see, it highlights that OA is more effective to decrease the switching cost. The superiority becomes significant for a large , which verifies our theoretical results nicely again.
7. Conclusion and future work
We have proposed a new dynamic regret with switching cost and a new analysis framework for both online algorithms and online convex optimization. We find that the switching cost significantly impacts on the regret yielded by OA methods, but does not have an impact on the regret yielded by OCO methods. Empirical studies have validated our theoretical result.
Moreover, the switching cost in the paper is measured by using the norm of the difference between two successive decisions, that is, . It is interesting to investigate whether the work can be extended to a more general distance measure function such as Bregman divergence or Mahalanobis distance . Specifically, if the Bregman divergence666See details in https://en.wikipedia.org/wiki/Bregman_divergence. is used, the switching cost is thus , where is a differentiable distance function. If the Mahalanobis distance777See details in https://en.wikipedia.org/wiki/Mahalanobis_distance. is used, the switching cost is thus , where is the given covariance matrix. We leave the potential extension as the future work.
Besides, our analysis provides regret bound for any given budget of dynamics . It is a good direction to extend the work in the parameter-free setting, where analysis is adaptive to the dynamics of environment. Some previous work such as (Zhang et al., 2018a) have proposed the adaptive online method and analysis framework. But, (Zhang et al., 2018a) works in the expert setting, not a general setting of online convex optimization. It is still unknown whether their method can be used to extend our analysis.
This work was supported by the National Key R & D Program of China 2018YFB1003203 and the National Natural Science Foundation of China (Grant No. 61672528, 61773392, and 61671463).
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Lemma 1 ().
Given any vectors , , , and a constant scalar , if
Denote , and . According to the optimality of , we have
Thus, we have
It completes the proof. ∎
Lemma 2 ().
For any , we have
According to the third-point identity of the Bregman divergence, we have
holds because holds for any vectors and . It completes the proof. ∎
Lemma 3 ().
Given and , if , we have
holds due to is -strongly convex, and holds due to the optimality of . Thus,
It completes the proof. ∎
Proof to Theorem 1:
holds because has -Lipschitz gradient. holds due to Lemma 1 by setting , , , , and . holds because that is -strongly convex, that is, . holds due to .
Choose . We have
Since it holds for any seqence , we finally obtain
It completes the proof. ∎
Proof to Theorem 2:
holds due to Lemma 1 by setting ,