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35
ShoreMT: A Scalable Storage Manager for the Multicore Era
 EXTENDING DATABASE TECHNOLOGY (EDBT)
, 2009
"... Database storage managers have long been able to efficiently handle multiple concurrent requests. Until recently, however, a computer contained only a few singlecore CPUs, and therefore only a few transactions could simultaneously access the storage manager's internal structures. This allowed stora ..."
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Cited by 23 (9 self)
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Database storage managers have long been able to efficiently handle multiple concurrent requests. Until recently, however, a computer contained only a few singlecore CPUs, and therefore only a few transactions could simultaneously access the storage manager's internal structures. This allowed storage managers to use nonscalable approaches without any penalty. With the arrival of multicore chips, however, this situation is rapidly changing. More and more threads can run in parallel, stressing the internal scalability of the storage manager. Systems optimized for high performance at a limited number of cores are not assured similarly high performance at a higher core count, because unanticipated scalability obstacles arise. We benchmark four popular opensource storage managers (Shore, BerkeleyDB, MySQL, and PostgreSQL) on a modern multicore machine, and find that they all suffer in terms of scalability. We briefly examine the bottlenecks in the various storage engines. We then present ShoreMT, a multithreaded and highly scalable version of Shore which we developed by identifying and successively removing internal bottlenecks. When compared to other DBMS, ShoreMT exhibits superior scalability and 24 times higher absolute throughput than its peers. We also show that designers should favor scalability to singlethread performance, and highlight important principles for writing scalable storage engines, illustrated with real examples from the development of ShoreMT.
More Robust Hashing: Cuckoo Hashing with a Stash
 IN PROCEEDINGS OF THE 16TH ANNUAL EUROPEAN SYMPOSIUM ON ALGORITHMS (ESA
, 2008
"... Cuckoo hashing holds great potential as a highperformance hashing scheme for real applications. Up to this point, the greatest drawback of cuckoo hashing appears to be that there is a polynomially small but practically significant probability that a failure occurs during the insertion of an item, r ..."
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Cited by 19 (5 self)
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Cuckoo hashing holds great potential as a highperformance hashing scheme for real applications. Up to this point, the greatest drawback of cuckoo hashing appears to be that there is a polynomially small but practically significant probability that a failure occurs during the insertion of an item, requiring an expensive rehashing of all items in the table. In this paper, we show that this failure probability can be dramatically reduced by the addition of a very small constantsized stash. We demonstrate both analytically and through simulations that stashes of size equivalent to only three or four items yield tremendous improvements, enhancing cuckoo hashing’s practical viability in both hardware and software. Our analysis naturally extends previous analyses of multiple cuckoo hashing variants, and the approach may prove useful in further related schemes.
Tight thresholds for cuckoo hashing via XORSAT
, 2010
"... We settle the question of tight thresholds for offline cuckoo hashing. The problem can be stated as follows: we have n keys to be hashed into m buckets each capable of holding a single key. Each key has k ≥ 3 (distinct) associated buckets chosen uniformly at random and independently of the choices ..."
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Cited by 18 (1 self)
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We settle the question of tight thresholds for offline cuckoo hashing. The problem can be stated as follows: we have n keys to be hashed into m buckets each capable of holding a single key. Each key has k ≥ 3 (distinct) associated buckets chosen uniformly at random and independently of the choices of other keys. A hash table can be constructed successfully if each key can be placed into one of its buckets. We seek thresholds ck such that, as n goes to infinity, if n/m ≤ c for some c < ck then a hash table can be constructed successfully with high probability, and if n/m ≥ c for some c> ck a hash table cannot be constructed successfully with high probability. Here we are considering the offline version of the problem, where all keys and hash values are given, so the problem is equivalent to previous models of multiplechoice hashing. We find the thresholds for all values of k> 2 by showing that they are in fact the same as the previously known thresholds for the random kXORSAT problem. We then extend these results to the setting where keys can have differing number of choices, and provide evidence in the form of an algorithm for a conjecture extending this result to cuckoo hash tables that store multiple keys in a bucket.
Efficient hashing with lookups in two memory accesses, in: 16th
 SODA, ACMSIAM
"... The study of hashing is closely related to the analysis of balls and bins. Azar et. al. [1] showed that instead of using a single hash function if we randomly hash a ball into two bins and place it in the smaller of the two, then this dramatically lowers the maximum load on bins. This leads to the c ..."
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Cited by 16 (3 self)
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The study of hashing is closely related to the analysis of balls and bins. Azar et. al. [1] showed that instead of using a single hash function if we randomly hash a ball into two bins and place it in the smaller of the two, then this dramatically lowers the maximum load on bins. This leads to the concept of twoway hashing where the largest bucket contains O(log log n) balls with high probability. The hash look up will now search in both the buckets an item hashes to. Since an item may be placed in one of two buckets, we could potentially move an item after it has been initially placed to reduce maximum load. Using this fact, we present a simple, practical hashing scheme that maintains a maximum load of 2, with high probability, while achieving high memory utilization. In fact, with n buckets, even if the space for two items are preallocated per bucket, as may be desirable in hardware implementations, more than n items can be stored giving a high memory utilization. Assuming truly random hash functions, we prove the following properties for our hashing scheme. • Each lookup takes two random memory accesses, and reads at most two items per access. • Each insert takes O(log n) time and up to log log n+ O(1) moves, with high probability, and constant time in expectation. • Maintains 83.75 % memory utilization, without requiring dynamic allocation during inserts. We also analyze the tradeoff between the number of moves performed during inserts and the maximum load on a bucket. By performing at most h moves, we can maintain a maximum load of O(hlogl((~og~og:n/h)). So, even by performing one move, we achieve a better bound than by performing no moves at all. 1
Realtime parallel hashing on the gpu
 In ACM SIGGRAPH Asia 2009 papers, SIGGRAPH ’09
, 2009
"... Figure 1: Overview of our construction for a voxelized Lucy model, colored by mapping x, y, and z coordinates to red, green, and blue respectively (far left). The 3.5 million voxels (left) are input as 32bit keys and placed into buckets of ≤ 512 items, averaging 409 each (center). Each bucket then ..."
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Cited by 15 (4 self)
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Figure 1: Overview of our construction for a voxelized Lucy model, colored by mapping x, y, and z coordinates to red, green, and blue respectively (far left). The 3.5 million voxels (left) are input as 32bit keys and placed into buckets of ≤ 512 items, averaging 409 each (center). Each bucket then builds a cuckoo hash with three subtables and stores them in a larger structure with 5 million entries (right). Closeups follow the progress of a single bucket, showing the keys allocated to it (center; the bucket is linear and wraps around left to right) and each of its completed cuckoo subtables (right). Finding any key requires checking only three possible locations. We demonstrate an efficient dataparallel algorithm for building large hash tables of millions of elements in realtime. We consider two parallel algorithms for the construction: a classical sparse perfect hashing approach, and cuckoo hashing, which packs elements densely by allowing an element to be stored in one of multiple possible locations. Our construction is a hybrid approach that uses both algorithms. We measure the construction time, access time, and memory usage of our implementations and demonstrate realtime performance on large datasets: for 5 million keyvalue pairs, we construct a hash table in 35.7 ms using 1.42 times as much memory as the input data itself, and we can access all the elements in that hash table in 15.3 ms. For comparison, sorting the same data requires 36.6 ms, but accessing all the elements via binary search requires 79.5 ms. Furthermore, we show how our hashing methods can be applied to two graphics applications: 3D surface intersection for moving data and geometric hashing for image matching.
Linear probing with constant independence
 In STOC ’07: Proceedings of the thirtyninth annual ACM symposium on Theory of computing
, 2007
"... Hashing with linear probing dates back to the 1950s, and is among the most studied algorithms. In recent years it has become one of the most important hash table organizations since it uses the cache of modern computers very well. Unfortunately, previous analyses rely either on complicated and space ..."
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Cited by 14 (2 self)
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Hashing with linear probing dates back to the 1950s, and is among the most studied algorithms. In recent years it has become one of the most important hash table organizations since it uses the cache of modern computers very well. Unfortunately, previous analyses rely either on complicated and space consuming hash functions, or on the unrealistic assumption of free access to a truly random hash function. Already Carter and Wegman, in their seminal paper on universal hashing, raised the question of extending their analysis to linear probing. However, we show in this paper that linear probing using a pairwise independent family may have expected logarithmic cost per operation. On the positive side, we show that 5wise independence is enough to ensure constant expected time per operation. This resolves the question of finding a space and time efficient hash function that provably ensures good performance for linear probing.
Succinct Data Structures for Retrieval and Approximate Membership
"... Abstract. The retrieval problem is the problem of associating data with keys in a set. Formally, the data structure must store a function f: U → {0, 1} r that has specified values on the elements of a given set S ⊆ U, S  = n, but may have any value on elements outside S. All known methods (e. g. ..."
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Cited by 13 (6 self)
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Abstract. The retrieval problem is the problem of associating data with keys in a set. Formally, the data structure must store a function f: U → {0, 1} r that has specified values on the elements of a given set S ⊆ U, S  = n, but may have any value on elements outside S. All known methods (e. g. those based on perfect hash functions), induce a space overhead of Θ(n) bits over the optimum, regardless of the evaluation time. We show that for any k, query time O(k) can be achieved using space that is within a factor 1 + e −k of optimal, asymptotically for large n. The time to construct the data structure is O(n), expected. If we allow logarithmic evaluation time, the additive overhead can be reduced to O(log log n) bits whp. A general reduction transfers the results on retrieval into analogous results on approximate membership, a problem traditionally addressed using Bloom filters. Thus we obtain space bounds arbitrarily close to the lower bound for this problem as well. The evaluation procedures of our data structures are extremely simple. For the results stated above we assume free access to fully random hash functions. This assumption can be justified using space o(n) to simulate full randomness on a RAM. 1
Deamortized Cuckoo Hashing: Provable WorstCase Performance and Experimental Results
"... Cuckoo hashing is a highly practical dynamic dictionary: it provides amortized constant insertion time, worst case constant deletion time and lookup time, and good memory utilization. However, with a noticeable probability during the insertion of n elements some insertion requires Ω(log n) time. Whe ..."
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Cited by 10 (3 self)
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Cuckoo hashing is a highly practical dynamic dictionary: it provides amortized constant insertion time, worst case constant deletion time and lookup time, and good memory utilization. However, with a noticeable probability during the insertion of n elements some insertion requires Ω(log n) time. Whereas such an amortized guarantee may be suitable for some applications, in other applications (such as highperformance routing) this is highly undesirable. Kirsch and Mitzenmacher (Allerton ’07) proposed a deamortization of cuckoo hashing using queueing techniques that preserve its attractive properties. They demonstrated a significant improvement to the worst case performance of cuckoo hashing via experimental results, but left open the problem of constructing a scheme with provable properties. In this work we present a deamortization of cuckoo hashing that provably guarantees constant worst case operations. Specifically, for any sequence of polynomially many operations, with overwhelming probability over the randomness of the initialization phase, each operation is performed in constant time. In addition, we present a general approach for proving that the performance guarantees are preserved when using hash functions with limited independence