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(1999)Conference PaperLinking and loading are the final steps in preparing a program for execution. This paper assesses issues concerning dynamic and static linking in traditional as well as single-address-space operating systems (SASOS). Related loading issues are also addressed. We present the dynamic linking model implemented in the Mungi SASOS and discuss its strengths and limitations. Benchmarking shows that dynamic linking in a SASOS carries significantly less overhead than dynamic linking in SGI`s Irix operating system. The same performance advantages could be achieved in Unix systems, if they reserved a portion of the address space for dynamically linked libraries, and ensured that each library is always mapped at the same address.
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(1999)Conference PaperMost modern wide-address computer architecture do not prescribe a page table format, but instead feature a software-loaded TLB, which gives the operating system complete flexibility in the implementation of page tables. Such flexibility is necessary, as to date no single page table format has been established to perform best under all loads. With the recent trend to kernelised operating systems, which rely heavily on mapping operations for fast data movement across address-spaces, demands on page tables become more varied, and hence less easy to satisfy with a single structure. This paper examines the issue of page tables suitable for 64-bit systems, particularly systems based on microkernels. We have implemented a number of candidate page table structures in a fast microkernel and have instrumented the kernel`s TLB miss handlers. We have then measured the kernel`s performance under a variety of benchmarks, simulating loads imposed by traditional compact address spaces (typical for UNIX systems) as well as the sparse address spaces (typical for microkernel-based systems). The results show that guarded page tables, together with a software TLB cache, do not perform significantly worse than any of the other structures, and clearly outperform the other structures where the address space is used very sparsely.
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(1998)Conference PaperWe present the accounting system used for backing store management in the Mungi single-address-space operating system. The model is designed such that all accounting can be done asynchronously to operations on storage objects, and hence without slowing down such operations. It is based on bank accounts from which rent is collected for the storage occupied by objects. Rent automatically increases as available storage runs low, forcing users to release unneeded storage. Bank accounts receive regular income. A taxation system is used to prevent excessive buildup of funds on underutilised accounts. The accounting system is mostly implemented at user level, with minimal support from the kernel. As a consequence, the accounting model can be changed without modifying the Mungi kernel.
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(1999)Journal ArticleThis paper examines the history of selenium pollution in Lake Macquarie, NSW, Australia, and three factors that may affect the redistribution and remobilisation of particle bound selenium: changes in redox state, bioturbation, and bioaccumulation by macrobenthos and bacteria. Sediment cores were taken from Nords Wharf, a relatively unpolluted area, and from Mannering Bay near the Vales Point coal-fired power station. The age profile at the unpolluted site seems to indicate that mild selenium pollution has been occurring for over 100 years, however, some mixing of the sediments has occurred. At the polluted site, the age profile indicated that major contamination has occurred in the last 30 years, due to an ash dam associated with nearby electric power generation facilities. The contamination chronology suggests that remobilisation and reduction processes have affected the selenium profile. Changing the redox state of Lake Macquarie sediment results in a release of selenium under oxidising conditions and immobilisation under reducing conditions. The sediment-bound selenium was associated with the operationally defined `organic/sulfide' fraction under reducing conditions, and as the redox potential increases this moves into the `exchangeable' and `iron/manganese oxyhydroxide' phases to a limited extent. Bioturbation by the animals Marphysa sanguinea and Spisula trigonella caused increases in the redox potential and pore water selenium concentrations in surfcial sediments relative to unbioturbated controls. Both animals accumulated significantly more selenium when exposed to contaminated sediment than when exposed to uncontaminated control sediments. Selenium concentrations in molluscs from Mannering Bay were all significantly higher than those collected from Nords Wharf. Most of the selenium in the mollusc tissues was found to be associated with the protein fraction. Selenium isolated from hydrolysed muscle tissue was not present as selenate or selenite but as selenomethionine and an unidentified compound. Seven types of bacteria were isolated from Lake Macquarie sediment. All seven isolates were able to transform selenite quantitatively to elemental selenium as evidenced by a red precipitate and identified by X-ray diffraction. Six isolates grew on media containing selenate but no elemental selenium was formed. Mass balances showed that for three isolates total selenium was conserved, selenate decreased and selenium (0; II-) increased indicating the production of non-volatile organic selenium compounds. For two isolates both total selenium and selenate decreased with no increase in selenium (0; II-), therefore, loss of selenium occurred from the media. Selenium is immobile in anoxic reduced sediments but may become available to benthos and fish as a consequence of sediment oxidation associated with bioturbation leading to bioaccumulation and transformation by macrobenthos and bacteria. These mechanisms can be invoked as possible transport pathways to explain the presence of selenium above background concentrations in preindustrial sediments, but further work dating the sediments in which elevated concentrations of selenium are found is needed to confirm this hypothesis.
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(1999)Journal ArticleMeasurements of selenium in sediments and benthic infauna of Lake Macquarie, an estuary on the east coast of Australia, indicate that sediments are a significant source of selenium in the lake's food web. Analysis of surficial sediment samples indicated higher selenium con- concentrations near what are believed to be the main industrial sources of selenium to the lake: a smelter and a power station. Sediment cores taken from sediments in Mannering Bay, near a power station at Vales Point, contained an average of 12 times more selenium in surficial sections than sediment cores from Nord's Wharf, a part of the lake remote from direct inputs of selenium. The highest selenium concentration found in Mannering Bay sediments (17.2 mg/g) was 69 times the apparent background concentration at Nord's Wharf (0.25 mg/g). Pore water concentrations in Mannering Bay were also high, up to 5 mg/l compared to those at Nord's Wharf which were below detection limits (0.2 mg/l). Selenium concentrations in muscle tissues of three benthic-feeding fish species (Mugil cephalus, Platycephalus fuscus, Acanthopagrus australis) were significantly correlated ( p < 0:05) with surficial sediment selenium concentra- tion. Selenium concentrations in polychaetes and molluscs of Mannering Bay were up to 58 times higher than those from Nord's Wharf. Two benthic organisms, the eunicid polychaete Marphysa sanguinea and the bivalve mollusc Spisula trigonella, were maintained at different densities in selenium-spiked sediments. Both animals accumulated selenium from the spiked sediment, confirming that bioaccumulation from contaminated sediments occurs. Collectively, these data suggest that benthic food webs are important sources of selenium to the fish of Lake Macquarie.
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(1998)Journal ArticleA recent publication says that the primary spelling of the word for the depth of water required to float a ship is draught, with draft as the secondary spelling. A survey of thirty practising naval architects was made to check the claim, and found that a large majority of Australian-educated naval architects spell it draft. The overall results are interesting and are given here.
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(1998)Journal ArticleThe two types of marine hydrometer commonly used by naval architects and surveyors measure two distinct, but related, properties. The properties have different units and are used for different purposes. Some users may not be aware of the two types of hydrometer, the properties measured, or the errors which can significantly affect the end result. The relationships between the measured properties are discussed, together with the specific applications of the two types of hydrometer. The conversion of measurements made on one type of hydrometer to the other type is given, with examples of the conversions. Application of the principles presented here will prevent confusion and ensure the use, rather than abuse, of the two types of marine hydrometer.