The simple resource control mechanisms in the basic design (Section 4.1.3) are sufficient for a system that always has abundant memory. Most systems, however, are not so over-supplied. Further, it seems likely that the performance of memory-intensive applications would be harmed by ineffective speculative execution. In this section, we describe our mechanism for controlling the memory overhead incurred by speculative processes. This mechanism enables practical speculative execution on systems which may experience memory contention.
It is difficult to control memory overhead while still enabling effective speculative execution because, unlike processor or disk bandwidth, it is usually not possible to determine whether the resource is actually being wasted, and can therefore be used without hurting system performance. For example, even if the memory mapped by all extant processes is much less than the total amount of memory in the system, there is always a chance that a page that contains file data will be re-accessed.
The previous user-level design relied on the TIP prefetching and caching manager [5]. TIP performs cost-benefit analysis to determine when allocating some memory for prefetching would be more beneficial than allowing the LRU file cache to retain that memory. TIP is not a complete memory management solution for speculative execution, however, because speculative processes need to allocate memory not only to hold prefetched data, but also to dynamically allocate memory and make modifiable copies of pages to which they have copy-on-write access. Furthermore, the benefit of allowing a speculative process to allocate memory for its own use depends entirely on whether it will subsequently be able to issue useful prefetches, which depends on factors not within its control, such as how often it will be preempted.
We preserve the principle idea of a cost-benefit framework but, rather than building a complicated system model to predict the benefit of each speculative allocation request, we propose a simpler reactive approach to controlling memory overhead. Specifically, we estimate the benefit that a speculative process has already provided by prefetching data, and the cost it has already incurred by its memory consumption. This allows us to estimate the current benefit or cost of each speculative process and, by disabling processes accordingly, restrict the overhead incurred by ineffective speculative execution.