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Memory Management in User Space in Java Develop PDF417 in Java Memory Management in User Space




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y = *x;. Yes Zero Latency Yes Non-Zero Latency No Fill Cache Line TLB Hit L1 Hit TLB Miss FIGURE 5-12. Cache Miss: Reading a Single Byte Can Cause a Cache Line Fill This may seem like PDF417 for Java nitpicking when you are working with processors that run at 3GHz, but the extra clock cycles add up, particularly if you are using large amounts of data.. Write Back, Write Through, and Prefetching Caches have differ Java barcode pdf417 ent modes of operation, and each CPU architecture has its own idiosyncrasies. The basic modes that they have in common are Write Back This is the highest-performance mode and the most typical. In write-back mode, the cache is not written to memory until a newer cache entry flushes it out or the software explicitly flushes it.

This enhances performance because the CPU can avoid extra writes to memory when a line of cache is modified more than once. Also, although cache lines may be written in random order, they may be flushed in sequential order, which may improve. 5 What Every Developer Should Know about the Kernel efficiency. This i pdf417 2d barcode for Java s sometimes called write combining and may not be available for every architecture.22 Write Through This is less efficient than write-back because it forces writes to complete to memory in addition to saving it in cache.

As a result, writes take longer, but reads from cache will still be fast. This is used when it s important for main memory and the cache to contain the same data at all times. Prefetching Some caches allow the processor to prefetch cache lines in response to a read request so that adjacent blocks of memory are read at the same time.

Reading in a burst of more than one cache line usually is more efficient than reading only one cache line. This improves performance if the software subsequently reads from those addresses. But if access is random, prefetching can slow the CPU.

Architectures that allow prefetching usually have special instructions allowing software to initiate a prefetch in the background to gain maximum parallelism.23 Most caches allow software to set the mode by regions so that one region may be write-back, another is write-through, and still another is noncacheable. Typically, these operations are privileged, so user programs never modify the write-back or write-through modes of the cache directly.

This kind of control usually is required only by device drivers. 5.6.

1.4 Programming Cache Hints Prefetching can be controlled by software through so-called cache hints with the madvise function. This API allows you to tell the operating system how you plan to use a block of memory.

There are no guarantees that the operating system will take your advice, but when it does, it can improve performance, given the right circumstances. To tell the OS that prefetching would be a good idea, you would use this pattern:. madvise( pointer, size, MADV_WILLNEED MADV_SEQUENTIAL);. 22. Write combinin g is similar to merging I/O requests in the I/O scheduler discussed earlier in the chapter. 23.

Some newer BIOSes allow you to enable or disable cache line prefetching at the system level.. Memory Management in User Space These two flags te ll the OS that you will be using the memory shortly and that you will be doing sequential access. Prefetching can be a liability if you are accessing data in a random fashion, so the same API allows you to tell the OS that prefetching is a bad idea. For example:.

madvise( pointer, tomcat PDF-417 2d barcode size, MADV_RANDOM );. The madvise functi on has other flags to suggest that flushing or syncing would be a good idea, but the msync function usually is more appropriate for this purpose. 5.6.

1.5 Memory Coherency Memory coherency refers to the unique problem that multiprocessor systems have in keeping their caches up to date. When one processor modifies a memory location in cache, the second processor will not see it until that cache is written back to memory.

In theory, if the second processor reads that location, it will get the incorrect value. In reality, modern processors have elaborate mechanisms in hardware to ensure that this doesn t happen. Under normal circumstances, this is transparent to software, particularly in user space.

In a Symmetric Multiprocessing System (SMP), the hardware is responsible for keeping the cache coherent between CPUs. Even in a single-processor system, memory coherency can be an issue because some peripheral hardware can take the place of other processors. Any hardware that can access system memory via Direct Memory Access (DMA) can read or write memory without the processor s knowledge.

Most PCI cards, for example, have DMA controllers. When a controller writes to system memory via DMA, there is a chance that some of those locations are sitting in the CPU cache. If so, the data in cache will be invalid.

Likewise, if the necessary data is sitting in cache when a device reads from memory via DMA, the device will get the wrong data. It is the job of the operating system (typically, a device driver) to manage the DMA transfers and the cache to prevent this. If the device driver allows mmap, it may be up to the application to manage the memory coherency.

When the data in cache is older than the data in memory, we say that it is stale. If the software initiates a DMA transfer from a device to RAM, the software must tell the CPU that the cached entries must be discarded. On some systems, this is called invalidating the cache entries.

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