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Straggler Mitigation at Scale [article]

Mehmet Fatih Aktas, Emina Soljanin
2019 arXiv   pre-print
Runtime performance variability at the servers has been a major issue, hindering the predictable and scalable performance in modern distributed systems. Executing requests or jobs redundantly over multiple servers has been shown to be effective for mitigating variability, both in theory and practice. Systems that employ redundancy has drawn significant attention, and numerous papers have analyzed the pain and gain of redundancy under various service models and assumptions on the runtime
more » ... ity. This paper presents a cost (pain) vs. latency (gain) analysis of executing jobs of many tasks by employing replicated or erasure coded redundancy. Tail heaviness of service time variability is decisive on the pain and gain of redundancy and we quantify its effect by deriving expressions for the cost and latency. Specifically, we try to answer four questions: 1) How do replicated and coded redundancy compare in the cost vs. latency tradeoff? 2) Can we introduce redundancy after waiting some time and expect to reduce the cost? 3) Can relaunching the tasks that appear to be straggling after some time help to reduce cost and/or latency? 4) Is it effective to use redundancy and relaunching together? We validate the answers we found for each of the questions via simulations that use empirical distributions extracted from a Google cluster data.
arXiv:1906.10664v2 fatcat:t62geuwic5bcjjhiffc5dft4mi

Simplex Queues for Hot-Data Download [article]

Mehmet Fatih Aktas, Elie Najm, Emina Soljanin
2018 arXiv   pre-print
In cloud storage systems, hot data is usually replicated over multiple nodes in order to accommodate simultaneous access by multiple users as well as increase the fault tolerance of the system. Recent cloud storage research has proposed using availability codes, which is a special class of erasure codes, as a more storage efficient way to store hot data. These codes enable data recovery from multiple, small disjoint groups of servers. The number of the recovery groups is referred to as the
more » ... ability and the size of each group as the locality of the code. Until now, we have very limited knowledge on how code locality and availability affect data access time. Data download from these systems involves multiple fork-join queues operating in-parallel, making the analysis of access time a very challenging problem. In this paper, we present an approximate analysis of data access time in storage systems that employ simplex codes, which are an important and in certain sense optimal class of availability codes. We consider and compare three strategies in assigning download requests to servers; first one aggressively exploits the storage availability for faster download, second one implements only load balancing, and the last one employs storage availability only for hot data download without incurring any negative impact on the cold data download.
arXiv:1804.06489v1 fatcat:ogt7qifwvzanhbypik3hmyotxe

Straggler Mitigation by Delayed Relaunch of Tasks [article]

Mehmet Fatih Aktas, Pei Peng, Emina Soljanin
2017 arXiv   pre-print
Redundancy for straggler mitigation, originally in data download and more recently in distributed computing context, has been shown to be effective both in theory and practice. Analysis of systems with redundancy has drawn significant attention and numerous papers have studied pain and gain of redundancy under various service models and assumptions on the straggler characteristics. We here present a cost (pain) vs. latency (gain) analysis of using simple replication or erasure coding for
more » ... er mitigation in executing jobs with many tasks. We quantify the effect of the tail of task execution times and discuss tail heaviness as a decisive parameter for the cost and latency of using redundancy. Specifically, we find that coded redundancy achieves better cost vs. latency tradeoff than simple replication and can yield reduction in both cost and latency under less heavy tailed execution times. We show that delaying redundancy is not effective in reducing cost and that delayed relaunch of stragglers can yield significant reduction in cost and latency. We validate these observations by comparing with the simulations that use empirical distributions extracted from Google cluster data.
arXiv:1710.00414v1 fatcat:ilwh4i5s5vgtbofvt7fr3t7x7u

Refractive Results Using a New Optical Biometry Device

Serdar Aktas, Hatice Aktas, Mehmet Tetikoglu, Hac Murat Sagdk, Fatih Özcura
2015 Medicine  
The aim of the study was to compare the measurements of optical (AL-Scan; Nidek Co., Ltd.) and ultrasonic (Echo Scan US-800; Nidek Co., Ltd.) biometry devices and to assess refractive results after cataract surgery. Eighty-one cataractous eyes of 81 patients were included in this study. Biometry was performed using the AL-Scan and an ultrasonic biometer (USB). Axial length (AL), keratometry (K) data, and intraocular lens (IOL) power calculations using the SRK/T formula were compared.
more » ... n analysis was used to assess the extent of agreement between AL-Scan and USB data in terms of AL measurement and IOL power calculation. The K measurements of the AL-Scan were compared to autorefractor data (Canon Autorefractor RK-F1). The AL-Scan assessed the AL as longer (average difference 0.06 ± 0.18 mm; ICC = 0.987; P < 0.001) and the IOL power as greater (average difference 0.19 ± 0.66 D; ICC = 0.964; P < 0.001) than the USB. The AL-Scan also measured average K values (average difference 0.25 ± 0.25 D; ICC = 0.985; P < 0.001) greater than those given by the autorefractor. The postoperative mean absolute error was +0.30 ± 0.04 D (minimum: -0.51 D, maximum +1.04 D). The postoperative mean K value change was 0.36 ± 0.29 D (P < 0.05). The differences between measurements afforded by the AL-Scan and USB may be clinically acceptable. Keratometric changes that develop after cataract operations compromise the attainment of good refractive outcomes.
doi:10.1097/md.0000000000002169 pmid:26632900 pmcid:PMC4674203 fatcat:5yba4s7anbaupfonv3mduheofu

Effective Straggler Mitigation: Which Clones Should Attack and When? [article]

Mehmet Fatih Aktas, Pei Peng, Emina Soljanin
2017 arXiv   pre-print
Redundancy for straggler mitigation, originally in data download and more recently in distributed computing context, has been shown to be effective both in theory and practice. Analysis of systems with redundancy has drawn significant attention and numerous papers have studied pain and gain of redundancy under various service models and assumptions on the straggler characteristics. We here present a cost (pain) vs. latency (gain) analysis of using simple replication or erasure coding for
more » ... er mitigation in executing jobs with many tasks. We quantify the effect of the tail of task execution times and discuss tail heaviness as a decisive parameter for the cost and latency of using redundancy. Specifically, we find that coded redundancy achieves better cost vs. latency and allows for greater achievable latency and cost tradeoff region compared to replication and can yield reduction in both cost and latency under less heavy tailed execution times. We show that delaying redundancy is not effective in reducing cost.
arXiv:1710.00748v1 fatcat:tkrje7xmkjatverun6usjy2fly

Simplex Queues for Hot-Data Download

Mehmet Fatih Aktas, Elie Najm, Emina Soljanin
2017 Performance Evaluation Review  
In cloud storage systems, hot data is usually replicated over multiple disks/servers in order to accommodate simultaneous access by multiple users as well as increase the fault tolerance of the system. Recent cloud storage research has proposed using availability codes, which is a special class of erasure codes, as a more storagee cient way to store hot data. ese codes enable data recovery from multiple, small disjoint groups of servers. e number of the recovery groups is referred to as the
more » ... lability and the size of each group as the locality of the code. Up till now, we have very limited knowledge on how code locality and availability a ect data access time. Data download from these systems involves multiple fork-join queues operating in-parallel, making the analysis of access time a very challenging problem. In this paper, we present an analysis of average data access time in storage systems employing simplex codes, which are an important, in certain sense optimal, class of availability codes. We generalize the analysis for codes with locality 2 and any degree of availability. Speci cally, using a queueing theoretic approach, we derive bounds and approximations on the average response time for two di erent Poisson request arrival models. We also compare two scheduling strategies for reduced access time and load balancing.
doi:10.1145/3143314.3078553 fatcat:hw4zaolfqnbn3oijperycijhma

Serum Levels of Omentin in End-Stage Renal Disease Patients

Aytekin Alcelik, Mehmet Tosun, Mehmet Fatih Ozlu, Mustafa Eroglu, Gulali Aktas, Eray Kemahli, Haluk Savli, Mehmet Yazici
2012 Kidney & Blood Pressure Research  
found to be correlated with creatinine (r = 0.333, p = 0.002).
doi:10.1159/000338796 pmid:22813935 fatcat:p5rbriarj5ehvbrdrqdet57mvu

FARKLI UZAKLIK HESAPLAMA YAKLAŞIMLARININ TOPSIS ÜZERİNDE KULLANILABİLİRLİĞİNİN İNCELENMESİ

Mehmet Kabak, Fatih Sağlam, Ahmet Aktaş
2017 Gazi Üniversitesi Mühendislik-Mimarlık Fakültesi Dergisi  
Çok kriterli karar verme yaklaşımlarından TOPSIS tekniği için farklı uzaklık ölçütleri önerilmiştir  Öklid uzaklığı dışında 6 farklı yeni uzaklık ölçütü tanıtılmıştır  İki örnek uygulama üzerinde alternatif sıralamalarının değişimi incelenmiştir
doi:10.17341/gazimmfd.300592 fatcat:x46ypvbkdbggzm5wmp7fvdx7ui

Comparison of Refractive Errors Measured by Plusoptix and Autorefractometer in Children
Çocukluk Çağında Plusoptix ve Otorefraktometre ile Ölçülen Refraksiyon Değerlerinin Karşılaştırılması

Serdar AKTAŞ, Fatih ÖZCURA, Hatice AKTAŞ, Mehmet TETİKOĞLU, Hacı Murat SAĞDIK
2015 Turkiye Klinikleri Journal of Ophthalmology  
KAYNAKLAR Serdar AKTAŞ ve ark. ÇOCUKLUK ÇAĞINDA PLUSOPTIX VE OTOREFRAKTOMETRE İLE ÖLÇÜLEN REFRAKSİYON DEĞERLERİNİN... Turkiye Klinikleri J Ophthalmol 2015;24(1)  ... 
doi:10.5336/ophthal.2014-40061 fatcat:6z37blpk2jh3pojituhtvfewni

Repeatability and Reproducibility of a New Partial Coherence Interferometer; AL-Scan Optic Biometer

Mehmet Tetikoglu, Haci Sagdik, Serdar Aktas, Fatih Ozcura
2016 Medicine Science  
The purpose of this study was to evaluate the intra-observer repeatability and inter-observer reproducibility of the ocular parameter measurements in cataract eyes using the new partial coherence interferometer, AL-Scan. Eighty-six eyes with cataracts were included in this prospective study. Axial length, average keratometry, anterior chamber depth, central corneal thickness, white-to-white distance and intraocular lens power calculation with the SRK-T formula were determined by two observers.
more » ... he measurements were repeated by the same observers using the same AL-Scan unit on the same eye approximately 2 weeks later, just before surgery. Inter-observer reproducibility was excellent, with very high interclass correlation coefficients (˃0.984) for all measured parameters. Standard deviations (Sw) and coefficients of variation (CV) of the repeated measurements were low, which demonstrated high intra-observer repeatability, except for the central corneal thickness and white-to-white distance measurements (Sw≤0.224 ∞CV≤ 1.072). The precision of the measurements obtained by the AL-Scan biometer is highly reliable and observer-independent.
doi:10.5455/medscience.2015.04.8372 fatcat:bclzmivv7nglpjle3kceocvb7m

Erythrocyte Membrane Fatty Acid Composition in Premenopausal Patients with Iron Deficiency Anemia

Mehmet Aktas, Mahfuz Elmastas, Fatih Ozcicek, Necmettin Yilmaz
2016 Journal of Oleo Science  
doi:10.5650/jos.ess15211 pmid:26876679 fatcat:gmbghfhaubfmtoenrqved6hjiu

N-terminal pro-B-type natriuretic peptide and insulin resistance in overweight and obese subjects

Aytekin Alcelik, Eyup Aytekin, Mehmet Tosun, Gülali Aktaş, Selim Ayhan, Serkan Ozturk, M. Fatih Ozlu, Haluk Savli, Mehmet Yazici
2012 Journal of Experimental and Clinical Medicine  
ÖZET Farklı obez popülasyonlarda B-tip natriüretik peptid (BNP) seviyelerini araştıran bazı çalışmalar yapılmıştır. Henüz tam açıklanamayan bir mekanizma ile obezlerde BNP düzeyleri sıklıkla düşük olarak saptanmaktadır. Bilinen kalp hastalığı olmayan bireylerde plazma NT-proBNP seviyeleri, obezite ve insülin direnci arasındaki ilişkiyi araştırmayı amaçladık. Çalışma popülasyonumuz iki gruptan oluşmaktaydı. Vücut kitle indeksi (VKİ) ≤25 kg/m 2 olanlar zayıf grubuna (LG, n=30), >25 kg/m 2 olanlar
more » ... ise fazla kilolu ve obez grubuna (OG, n=78) alındı. Ciddi hipertansiyonu, kalp yetmezliği, iskemik kalp hastalığı, böbrek veya karaciğer yetmezliği, malignitesi olan hastalar, gebeler ve 65 yaş üstü hastalar çalışmaya alınmadı. Zayıf hastaların grubunda NT-proBNP ile kilo (r=-0,379, p=0,004) ve VKİ (r=-0,286, p=0,030) arasında negatif bir ilişki varken, obez grupta NT-proBNP ve VKİ arasında böyle bir ters korelasyon saptanmadı. Obezite seviyelerinin belirteçlerini saptamak için çok değişkenli lojistik regresyon analizleri yaptık. Bu belirteçler ile obezite arasında anlamlı ilişki saptanmadı. İnsülin seviyelerinin obezite ile güçlü bir ilişkisi vardı ancak yine de bu ilişki istatistiksel anlamlılık düzeylerine ulaşamıyordu (p=0,054). Zayıf hasta grubunda NT-proBNP ve VKİ arasında ters korelasyon vardı. Aynı grupta NT-proBNP ve homeastatik modelde değerlendirme (HOMA) ve insülin seviyeleri (r=-0,035, p=0,789) arasında korelasyon yoktu, ancak obez grupta BNP ve HOMA (r=-0,219, p=0,009) ve insulin seviyeleri (r=-0,252, p=0,002) arasında istatistiksel olarak anlamlı ters ilişki vardı. Irk ve etnik köken bu farklılığa katkıda bulunuyor olabilir.
doi:10.5835/jecm.omu.29.03.004 fatcat:77yodgvf35f2xasupu3hilb2um

Evaluation of oxidative stress in sheep infected with Psoroptes ovis using total antioxidant capacity, total oxidant status, and malondialdehyde level

Mustafa Sinan Aktas, Fatih Mehmet Kandemir, Akin Kirbas, Basak Hanedan, Mehmet Akif Aydin
2017 Journal of Veterinary Research  
Introduction: The study aimed at evaluating oxidative stress using malondialdehyde (MDA), total antioxidant capacity (TAC), total oxidant status (TOS), and oxidative stress index (OSI) markers in sheep naturally infected with Psoroptes ovis (Acari). Material and Methods: The study was performed on 40 sheep divided into two equal groups: a healthy group (group I) and a group naturally infected with Psoroptes ovis (group II). The sera were obtained by centrifuging blood samples collected from the
more » ... vena jugularis and serum MDA level changes in the samples were measured spectrophotometrically. Commercially available test kits were used for the measurement of TAC and TOS levels. The percentage ratio of TOS level to TAC level was accepted as OSI. Results: The serum malondialdehyde, total oxidant status levels, and oxidative stress index increased significantly (P < 0.01) in group II, while the serum total antioxidant capacity levels decreased significantly (P < 0.01) in this group. Negative correlations between total antioxidant capacity and total oxidant status and total antioxidant capacity and malondialdehyde, and a positive correlation between total oxidant status and malondialdehyde were found in infected sheep. Conclusion: The obtained results indicated the relationship between oxidant/antioxidant imbalance and Psoroptes ovis infection in sheep. Their MDA, TAC, TOS, and OSI markers may be used to determine the oxidative stress in natural infections with Psoroptes ovis.
doi:10.1515/jvetres-2017-0025 pmid:29978073 pmcid:PMC5894390 fatcat:ejs6du4sivboxjotb6oxn4q2um

Evaluation of arginase activity, nitric oxide and oxidative stress status in sheep with contagious agalactia

Basak Hanedan, Akin Kirbas, Fatih Mehmet Kandemir, Mustafa Sinan Aktas, Ahmet Yildiz
2017 Acta Veterinaria Hungarica  
It is known that inflammatory organ damages due to various agents, such as microorganisms including mycoplasmas, lead to oxidative stress. Nitric oxide (NO) functions as an antimicrobial agent, and arginase decreases proinflammatory cytokine release. There are very few studies on arginase activity, NO level and oxidative stress status in mycoplasmal infections. Therefore, the aim of this study was to evaluate erythrocyte arginase activity, plasma NO level and oxidative stress status in sheep
more » ... h contagious agalactia. The study material consisted of 10 healthy sheep and 14 sheep with contagious agalactia characterised by mastitis, arthritis and keratoconjunctivitis. Erythrocyte arginase activity, plasma NO, malondialdehyde (MDA), total oxidant capacity (TOC) and total antioxidant capacity (TAC) levels were measured. Significant decreases in erythrocyte arginase activity and plasma TAC level (P < 0.001), and significant increases in plasma NO, MDA and TOC levels (P < 0.001) were found in the diseased sheep as compared with the healthy animals. This study suggests that contagious agalactia may cause oxidative stress due to increased plasma MDA and TOC levels and decreased plasma TAC levels, and that the decrease in erythrocyte arginase activity and increase in plasma NO level may contribute to the elimination of mycoplasmal agents causing contagious agalactia.
doi:10.1556/004.2017.037 pmid:28956486 fatcat:awewcynscrftxmqthsswpojwne

Effect of Corneal Incision Enlargement on Surgically Induced Astigmatism in Biaxial Microincision Cataract Surgery

Mehmet Tetikoğlu, Celal Yeter, Fırat Helvacıoğlu, Serdar Aktaş, Hacı Murat Sağdık, Fatih Özcura
2016 Türk Oftalmoloji Dergisi  
Objectives: To evaluate surgically induced astigmatism (SIA) in biaxial microincision cataract surgery with enlargement of one corneal incision during intraocular lens implantation (IOL). Materials and Methods: Data from 683 eyes with cataract that underwent biaxial microincision cataract surgery and IOL were ation. There were 83 eyes with 1.6 mm corneal incisions (group 1) and 200 eyes in each of the 2, 2.4, and 2.8 mm corneal incision groups (groups 2, 3 and 4, respectively). SIA was assessed
more » ... using preoperative and postoperative keratometric values at one month. Results: The mean magnitude of SIA was 0.83±0.4 D in group 1, 0.93±0.5 D in group 2, 1.03±0.6 D in group 3 and 1.04±0.7 D in Conclusion: Biaxial microincision cataract surgery with an incision size of 1.6 mm resulted in the least SIA. Enlargement of the and dissemination of IOLs which can be inserted through small corneal incisions, biaxial microincision cataract surgery will be the best choice to prevent SIA and increase visual acuity.
doi:10.4274/tjo.52386 pmid:27800270 pmcid:PMC5076300 fatcat:pmpp67tqvfe7zbwfhdc2t5aji4
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