• Journal of Ship and Ocean Technology
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• v.10 no.2
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• pp.37-44
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• 2006

The erection in shipbuilding is the process to assemble all the blocks one by one in certain order and requires more leveled and efficient schedule than other processes do. However, erection schedule includes too many constraints to be systemized with simple programs and constraints are changed frequently. These difficulties make it rare to find automatic erection schedule generation system with load leveling ability. In this paper, a CSP (Constraint Satisfaction Problem)-based load leveling algorithm using a maximum load diminution technique is proposed and applied to the block erection scheduling of a dock in a shipyard. The result shows that it performs better than currently used scheduling method based on empirical logics. The maximum load of welding length and crane usage are reduced by 31.63% and 30.00% respectively. The deviation of resource usage amount also decreases by 8.93% and 7.51%.

• Journal of the Korea Society of Computer and Information
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• v.20 no.12
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• pp.1-8
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• 2015

Wear leveling techniques have been studied to prolong the lifetime of NAND flash memory. Most of studies have used Program/Erase(P/E) cycles as wear index for wear leveling. Unfortunately, P/E cycles could not predict the real lifetime of NAND flash blocks. Therefore, these algorithms have the limited performance from prolonging the lifetime when applied to the SSD. In order to apply the real lifetime, wear leveling algorithms, which use raw Bit Error Rate(rBER) as wear index, have been studied in recent years. In this paper, we propose CrEWL(Cold data identification using raw Bit error rate in Wear Leveling), which uses rBER as wear index to apply to the real lifetime. The proposed wear leveling reduces an overhead of garbage collections by using HBSQ(Hot Block Sequence Queue) which identifies hot data. In order to reduce overhead of wear leveling, CrEWL does not perform wear leveling until rBER of the some blocks reaches a threshold value. We evaluate CrEWL in comparison with the previous studies under the traces having the different Hot/Cold rate, and the experimental results show that our wear leveling technique can reduce the overhead up to 41% and prolong the lifetime up to 72% compared with previous wear leveling techniques.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.13 no.2
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• pp.315-320
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• 2009

Since flash memory can not be overwritten on updating data, new data are updated in new area and old data should be invalidated and erased for garbage collection. With develop of flash memory technology, capacity of flash memory is rapidly increasing. It increases rapidly execution time of CPU to search an entire flash memory of large capacity when choosing the block to erase in garbage collection. To solve the problem that is increasing execution time of CPU, flash memory is partitioned into several groups, the block to erase in garbage collection is searched within the corresponding group. In workload of access locality, we enhanced wear-leveling within group by allocating hot data to hot group and cold data to cold group respectively and enhanced wear-leveling among groups by exchanging periodically hot group and cold group.

• Journal of the Korea Society of Computer and Information
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• v.21 no.5
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• pp.1-10
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• 2016

Magnetic disks have been used for decades in auxiliary storage devices of computer systems. In recent years, the use of NAND flash memory, which is called SSD, is increased as auxiliary storage devices. However, NAND flash memory, unlike traditional magnetic disks, necessarily performs the erase operation before the write operation in order to overwrite data and this leads to degrade the system lifetime and performance of overall NAND flash memory system. Moreover, NAND flash memory has the lower endurance, compared to traditional magnetic disks. To overcome this problem, this paper proposes EPET (Enhanced Prediction and Elapsed Time) wear leveling technique, which is especially efficient to portable devices. EPET wear leveling uses the advantage of PET (Prediction of Elapsed Time) wear leveling and solves long-term system failure time problem. Moreover, EPET wear leveling further improves space efficiency. In our experiments, EPET wear leveling prolonged the first bad time up to 328.9% and prolonged the system lifetime up to 305.9%, compared to other techniques.

• Journal of KIISE:Computing Practices and Letters
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• v.15 no.5
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• pp.322-330
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• 2009

In order to utilize NAND flash memory as storage media which does not allow update-in-place and limits the number of block erase count, various garbage collection policies supporting wear-leveling have been investigated. Conventional garbage collection policies require cleaning-index calculation for the entire blocks to choose a block to be garbage-collected to support wear-leveling whenever a garbage collection is required, which results in performance degradation of system. This paper proposes a garbage collection policy which supports wear-leveling using a threshold value, which is in fact a variance of erase counts and by the maximum erase count of all blocks, without calculating the cleaning-index. During garbage collection, the erase cost is minimized by using the Greedy Policy if the variance is less than the threshold value. It achieves wear-leveling by excluding the block with the largest erase count from erase target blocks if the variance is larger than threshold value. The proposed scheme shows that a standard deviation approaches to zero as the erase count of blocks approaches to its upper limit and the measured speed of garbage collection is two times faster than the conventional schemes.

• Journal of Digital Contents Society
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• v.14 no.1
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• pp.51-57
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• 2013

Storage device based on Flash Memory have many attractive features such as high performance, low power consumption, shock resistance, and low weight, so they replace HDDs to a certain extent. An Storage device based on Flash Memory has FTL(Flash Translation Layer) which emulate block storage devices like HDDs. A garbage collection, one of major functions of FTL, effects highly on the performance and the lifetime of devices. However, there is no de facto standard for new garbage collection algorithms. To solve this problem, we propose File Clustering Algorithm. File Clustering Algorithm respect to update page from same file at the same time. So, these are clustered to same block. For this mechanism, We propose Page Allocation Policy in FTL and use MIN-MAX GAP to guarantee wear leveling. To verify the algorithm in this paper, we use TPC Benchmark. So, The performance evaluation reveals that the proposed algorithm has comparable result with the existing algorithms(No wear leveling, Hot/Cold) and shows approximately 690% improvement in terms of the wear leveling.

• KIISE Transactions on Computing Practices
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• v.21 no.2
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• pp.126-131
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• 2015

As flash-based storage systems have been actively employed in large-scale servers and data centers, reliability has become an indispensable element. One promising technique for enhancing reliability is static wear-leveling, which distributes erase operations evenly among blocks so that the lifespan of storage systems can be prolonged. However, increasing the capacity makes the processing overhead of this technique non-trivial, mainly due to searching for blocks whose erase count would be minimum (or maximum) among all blocks. To reduce this overhead, we introduce a new randomized block selection method in static wear-leveling. Specifically, without exhaustive search, it chooses n blocks randomly and selects the maximal/minimal erased blocks among the chosen set. Our experimental results revealed that, when n is 2, the wear-leveling effects can be obtained, while for n beyond 4, the effect is close to that obtained from traditional static wear-leveling. For quantitative evaluation of the processing overhead, the scheme was actually implemented on an FPGA board, and overhead reduction of more than 3 times was observed. This implies that the proposed scheme performs as effectively as the traditional static wear-leveling while reducing overhead.

• Korean Management Science Review
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• v.13 no.1
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• pp.1-12
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• 1996

This paper deals with production scheduling of block assembly shop in shipbuilding company. The Objectives of the schedule are leveling of daily total workload and fair assignment of workload to each department of the shop. Optimizing the objectives, we consider some restrictions which consisit of block due dates, workability of each department, and space restriction. We formulate the problem in mathematical form. But the model is too complex to find the optimal solution (the problem is NP-complete) and we use the genetic algorithm to find a good solution.

• Journal of the Korean Institute of Intelligent Systems
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• v.20 no.6
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• pp.761-767
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• 2010

In the case of NAND flash memory, a whole block needs to be erased for update operations because update-in- place operations are not supported in NAND flash memory. Blocks of NAND flash memory have the limited erasure cycles, so frequently updated data (hot data) easily makes blocks worn out. As the result, the capacity of NAND flash memory will be reduced by hot data. In this paper, we propose a wear-leveling algorithm by discriminating hot and cold data based on the update patterns of data. When we applied this scheme to NAND flash memory, we confirmed that the erase counts of blocks became more uniform by mapping hot data to a block with a low erase count and cold data to block with a high erase count.

• IEMEK Journal of Embedded Systems and Applications
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• v.12 no.3
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• pp.185-192
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• 2017

Due to the characteristics of low power, high durability and high density, NAND flash memory is being heavily used in various type of devices such as USB, SD card, smart phone and SSD. On the other hand, because of another characteristic of flash cell with the limited number of program/erase cycles, NAND flash memory has a short lifetime compared to other storage devices. To overcome the lifetime problem, many researches related to the wear leveling have been conducted. This paper presents a method called a TCB (Tracking Cold Blocks) using more reinforced constraint conditions when classifying cold blocks than previous works. TCB presented in this paper keeps a MCT (Migrated Cold block Table) to manage the enhanced classification process of cold blocks, with which unnecessary migrations of pages can be reduced much more. Through the experiments, we show that TCB reduces the overhead of wear leveling by about 30% and increases the lifetime up to about 60% compared to BET and BST.