minimizing stall time in single disk susanne albers, naveen garg, stefano leonardi, carsten witt...
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Minimizing Stall Time in Single Disk
Susanne Albers, Naveen Garg, Stefano Leonardi, Carsten
WittPresented by Ruibin Xu
Introduction Prefetching and caching are
powerful techniques for increasing performance in disk systems
Prefetching: load memory blocks into the cache before the actual references (needs to evict blocks simultaneously)
Caching: maintain the most frequently accessed blocks in cache
Introduction
Both techniques have been studied extensively, but separately
Now look at them in an integrated manner
Focus on the offline problem
The problem definition
Assume all blocks reside on one disk The cache size is k Serving a request takes one time unit Fetching a block takes F time units Given a request sequence σ = r1, … ,
rn, how to schedule the prefetching to minimize the total stall time
An example
•k = 4
•F = 5
•Blocks a, b, c and d are initially in the cache
The minimum stall time is 3
Big question
Cao et. al. designed a 2-approximation algorithm.
Can this problem be solved exactly in polynomial time?
Yes, this paper answers this quesiton
The idea Use linear programming At first thought, needs to prove that
the optimum solution is integral by arguing that all vertices of the corresponding polytope are integral By showing that the constraint matrix is
total unimodular (ex. Bipartite matching)
By combinatorial argument(ex. Matching and matroid polytopes)
Main novelty
At second thought, the polytope corresponding to the LP to this problem has nonintegral vertices
Now if we can show that any solution to the LP can be written as a convex combination of (polynomially many) integral solutions, ……
The roadmap
1. Construct the LP2. Solve the LP3. Find the convex decomposition to
integral solutions
The LP formulation
This is a 0-1 LP The length of the request sequence
is n The cache size is k The fetching time is F The cache initially contains k blocks
never requested in the sequence
The variables of the LP
Consider all the intervals of the request sequence of length at most F : interval I = (i, j) of length |I|=j – i – 1, i = 0, … , n-1, j = 1, … , n, i < j
The variables of the LP Associate each interval I with an indicator
variable X(I) where X(I) =1 indicates a prefetch starting after request i and ending before request j and X(I) =0 indicates no prefetch is performed in this interval
With each interval I and distinct block a, associate variable fI,a ( eI,a), which is 1 if block a is fetched (evicted) in interval I and 0, otherwise
The objective func. of the LP The prefetch occuring in interval I
has a stall time F - |I|
Thus the objective function is
The constraints of the LP There are 7 kinds of constraints
A definition: an interval (a, b) is contained in an interval (c, d) if c ≤a and d ≥b, denoted by (a, b) (c, d)
The 1st constraint To ensure that two prefetches are
not performed simultaneously
The 2nd constraint For any interval, the total amount of
fetch should be exactly equal to the total amount of eviction and this value should not exceed the value of the interval
The 3rd constraint A block should be in cache when it is
referenced After each reference to a block, the block
is in cache. It can then be evicted at most once up until the next reference to that block, and if it is, it must be also be fetched back prior to that next reference
The 4th and 5th constraint To ensure that every block is in cache
at its first reference, the total fetch of a block on intervals before its first reference should be 1 and the total evict of the block on these intervals should be 0
The 6th constraint A block is not evicted for more than 1
unit after its last reference
The last constraint On each request, the requested block is
neither prefetched nor evicted
And
Solving the LP relaxation
First solve the LP relaxation. If we get an integral solution, we are done.
If not, find the convex combination
Modify the intervals
The goal: to obtain a total order of intervals
An interval I1 = (i1, j1) is properly contained in interval I2 = (i2, j2) iff i1 > i2 and j1 < j2
We don’t want any interval is properly contained in any interval
Modify the intervals
•For each pair of nested intervals, remove one of them and add two new intervals
Order the intervals
Now we can order the intervals by increasing starting points;
If two intervals have the same start point, then they are ordered by increasing end-points
Properties of the optimum sol.
Let C denote the cache configuration after we have performed the fetches and evicts corresponding to the first i intervals; let I be the (i+1)-st interval
There exists an optimum solution for which the next two claims are satisfied
Properties of the optimum sol. Claim 1: In interval I, we fetch the
block that is not completely in C and whose next reference is earliest
Claim 2: In interval I, we evict the block which is partially or completely in C whose next reference is furthest
Both claims can be proven by contradiction
Properties of the optimum sol.
The amount of fetch of a block prescribed by claim 1 might be less than x(I). In this case, we apply the same rule to fetch another block in I
The same holds for the case of evictions
Another view of the process of fetching/evicting Define the distance of interval I
View the process of fetching/evicting as a process in time by associating the time interval [dist(I), dist(I)+x(I)) with interval I
Another view of the process of fetching/evicting
There is a unique interval associated with each time instant
Also associate a unique fetch/evict with each time instant
Properties of the optimum sol.
From claim 1&2 and the ordering of fetches/evicts within an interval, it follows that a block a is fetched continuously till it is fully in cache
But the eviction of a could be interrupted before it is completely out of cache
Properties of the optimum sol.
Consider the fetches/evictions of a block a between two consecutive references to a
Lemma 1. Every interruption in the eviction of a is for some integral time units
Properties of the optimum sol. A block a is partially fetched/evicted if
the total extent to which a is fetched/evicted between two consecutive references is strictly less than 1
Lemma 2. If a is partially fetched/evicted, then the fetch of a begins some integral time units after the start of its eviction
Properties of the optimum sol.
Lemma 3. If a is evicted at time t and referenced again, then there is a time t’ = t + i, for some integer i, at which a is fetched back
The convex decomposition
Let t be in the range [0, 1) and let ti = i + t for every integer i, 0 ≤ i ≤ x(I)
Claim 3. Let t1, t2 be two time instants such that t2 = t1 + i for some positive integer i, and let I1, I2 be the intervals associated with these time instants. Then I1 and I2 are disjoint.
The convex decomposition
Lemma 4. For any time t in [0,1), the set of intervals that correspond to ti forms a feasible solution
Note that each solution is obtained not for just one value of t but for a range of values, say for all t in the range [a, b]. We associate a weight b – a in the decomposition.
Conclusion
An optimum prefetching/caching schedule for a single disk can be computed in polynomial time
Open problem
Now the problem can be solved exactly in polynomial time by using LP, Does there exist a combinatorial, polynomial time algorithm?
Yes, by using multicommodity network flows
The roadmap
1. Construct the LP2. Solve the LP
3. Find the convex decomposition to integral solutions
1. Construct the multicommodity network2. Solve the network
Problem No combinatorial polynomial-time
algorithm for computing non-integral min-cost flow is known
But we know an approximation algorithm: for any ε ≥ 0, δ ≥ 0, the algorithm computes a flow such that a fraction of at least 1 - ε of each demand in the network is satisfied and the cost of the flow is at most (1 + δ ) times the optimum
The network Given a request sequence of
length n, construct a network with n+1 commodities
Associate each request σ(i) with a commodity i, which has a source si, a sink ti and a demand di = 1
For each request σ(i) , introduce two vertices xi and x’i
An example network
Sketch of the network for request sequence abcbc and F=2
The problem of previous network
The construction allows a flow algorithm to saturate more than one of the edges that correspond to fetches executed simultaneously
Needs to make sure at most one fetch operation is executed at any time
Solution
Split the “super edge” (si, xj) into several parts and add one more commodity
For any l, 1≤ l ≤ n-1, let [l, l+1) be the time interval starting at the service of σ(l) and ending immediately before the service of σ(l+1)
Solution
For any fixed i and j, with 1 ≤ i ≤ n, and pi+1 ≤ j < i, introduce vertices vij
l and wijl where l = j, … , min{j+F,
i} -1 For any fixed i , with 1 ≤ i ≤ n,
introduce vertices viii-1 and wii
i-1
How to connect? How to assign cost and capacity?
Solution Now add the (n+1)-st commodity Let fl be the number of prefetches
whose execution overlaps with [l, l+1)
Commodity n+1 has a source sn+1, a sink tn+1 and a demand dn+1
Solution
The flow from sn+1 to tn+1 is routed through the edges (vij
l , wijl ) and
newly introduced “subsinks” tn+1l, 1
≤ l ≤ n-1
How to connect? How to assign cost and weight?
Optimal flows
Any feasible integral flow of cost C in the network correspond to a feasible prefetching/caching schedule with stall time C for σ, and vice versa
A non-integral flow correspond to a fractional prefetching/caching schedule
Apply the approximation algo.
Unfortunately, the flow computed by the algorithm does not correspond to a feasible fractional prefetching/caching schedule
It is possible that(1) more than one block is fetched at any time and (2)blocks are not completely in cache when requested
Apply the approximation algo.
The solution is to choose ε and δ properly and modify the flow
Choose ε=1/(4F2n3) and δ=1/(3nF)
Let Φ be the flow returned by the approximation algorithm
Apply the approximation algo.
The flow out of each source si, i={1,…n}, is lower bounded by 1-ε. Moreover, commodity n+1 might lack an amount of εdn+1≤ εFn2
Let ρ= 1-ε- εdn+1 , transform the flow Φ into a uniform flow Φ’ which directs exactly ρ units of flow from si to ti
Apply the approximation algo.
The flow Φ’ corresponds to a fractional solution in which all blocks have size ρ and the number of cache slots is upper bounded by k/ ρ
We can interpret the fractional solution to Φ’ as a convex combination of integral ρ-solution
Apply the approximation algo.
Let the cost of convex combination of ρ-solutions be C, we can prove that C≤OPT+1/3
By increasing the block size from ρ to 1, we obtain the integral solutions. Let the cost of convex combination of integral solutions be C’, we can prove that C’<OPT+1
Apply the approximation algo.
Also, it can be proven that no integral component of the convex composition does hold more than k blocks in cache concurrently
Therefore, the convex combination contains at least one integral solution with optimal costs.
Conclusion
An optimal solution can be computed by a combinatorial algorithm in polynomial time
The running time is O*(n18)