![]() ![]() Therefore, the hardware design of Fully Associative is complex, requires an address comparator for each Cache Line, and is very expensive, so Fully Associative is not suitable for CPU caching. If a memory address is given, knowing whether it exists in the cache requires parallel comparison of the memory addresses in all Cache Lines. Fully Associativeįully Associative means that data from any memory address can be cached in any of the Cache Lines. Since the size of the Cache is much smaller than the memory, an address association algorithm is needed that can allow data in memory to be mapped to the Cache. The data placement policy of the Cache determines where blocks of data in memory will be copied to the CPU Cache. If it does, it is a cache hit, otherwise it loads the entire Cache Line from the second level cache.The CPU assumes that if we currently need data at a certain address, it is likely that we will immediately access its neighboring address, i.e., memory accesses tend to be localized, and once the cache block is loaded into the cache, the read instruction can proceed normally. The first-level data cache checks to see if it has a cache block corresponding to this memory address. When the CPU sees an instruction to read memory, it passes the memory address to the first-level data cache. For example, if L1 has a 32KB data cache, it has 32KB /64B = 512 Cache Lines. The CPU Cache and memory exchange data in cache blocks Cache Line, and the size of the Cache Line in today’s mainstream CPUs is 64Bytes, which is the smallest unit of data the CPU can get from memory. Before explaining these two issues, it is important to understand how the CPU reads and writes caches. The gradual increase in the number of cache levels also introduces two more important issues: one is the cache access hit problem, and the other is the cache update consistency problem. L1, L2, and 元 are faster and more costly the closer they are to the CPU, and slower the farther they are from the CPU.Each CPU core has its own 256KB L2 cache, while the 6MB 元 cache is shared by all CPU cores.L2 cache and 元 cache do not distinguish between instruction and data. Each CPU core has 64KB of L1 cache, of which 32KB is instruction cache and the other 32KB is data cache.Take the 8265U I use as an example, its cache structure is as follows. Most of today’s CPUs have three levels of cache. It can be seen that the CPU is very slow to read and write data directly to the main memory and spends most of its time waiting for data transfer, so the CPU cache structure has been introduced. Some sequential memory accesses may be more efficient but also have some latency, while those random memory accesses consume more time. On top of that, since main memory uses capacitors to store information, it needs to periodically refresh what it stores in order to prevent information loss due to natural discharge, which can also introduce additional wait times. For example, due to the transfer characteristics of the level protocol, a certain amount of signal stabilization time is required before a row is selected, a column is selected, and reliable data is fetched. When we access memory, we encounter many fragmented wait cycles. The performance gap between CPU and main memory is called the von Neumann bottleneck. Access to Memory takes approximately 200 clock cycles.Accessing the 元 Cache takes about 50 clock cycles.Access to L2 Cache takes about 15 clock cycles.It takes about 4 clock cycles to access the L1 Cache. ![]() It takes 1 clock cycle to access CPU registers.As the performance gap between the CPU and main memory gradually increased, L2 and 元 caches were added gradually, and the access speed between CPU and Cache and Memory was roughly as follows. Due to the large performance gap between the CPU and main memory, the CPU had to wait a long time to read and write data, so an SRAM cache memory, called L1 cache, had to be inserted between the CPU registers and the main memory. ![]() The memory hierarchy of early computer systems had only three levels: CPU registers, DRAM main memory, and disk storage. This article will introduce the CPU cache system and how to use memory barriers for cache synchronization. On modern CPUs (most of them), all memory accesses need to go through layers of cache, and understanding the CPU cache update coherency issues can be of great help in designing and debugging our programs.
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