The advantage of this design is that we can continue to use small fixed size pages with standard TLBs. Since PTPCs use traditional page table structures and page sizes, they are very simple to implement in hardware and require minimal operating system modifications.
Our simulations show that the addition of a PTPC to a system with a TLB can reduce miss handling costs by nearly an order of magnitude. Citation Wu, Michael and Zwaenepoel, Willy. Type Technical report. Rights You are granted permission for the noncommercial reproduction, distribution, display, and performance of this technical report in any format, but this permission is only for a period of forty-five 45 days from the most recent time that you verified that this technical report is still available from the Computer Science Department of Rice University under terms that include this permission.
Therefore, let programmer write the code in virtual not real memory and let the memory management unit handle the conversion. Add a comment. Active Oldest Votes. Let me explain all these things step by step. Why TLB Translation Look Aside Buffer The thing is that page table is stored in physical memory , and sometimes can be very large, so to speed up the translation of logical address to physical address , we sometimes use TLB , which is made of expensive and faster associative memory , So instead of going into page table first, we go into the TLB and use page number to index into the TLB , and get the corresponding page frame number and if it is found, we completely avoid page table because we have both the page frame number and the page offset and form the physical address.
Page Fault Occurs when the page accessed by a running program is not present in physical memory. Cache Hit Cache Memory is a small memory that operates at a faster speed than physical memory and we always go to cache before we go to physical memory. Cache Miss It is only after when mapping to cache memory is unable to find the corresponding block block similar to physical memory page frame of memory inside cache called cache miss , then we go to physical memory and do all that process of going through page table or TLB.
So the flow is basically this 1. If its a cache miss , go to step 3. End Note The flow I have discussed is related to virtual cache VIVT faster but not sharable between processes , the flow would definitely change in case of physical cache PIPT slower but can be shared between processes.
Improve this answer. Sumeet Sumeet 7, 2 2 gold badges 21 21 silver badges 44 44 bronze badges. That was really helpful.
I don't think the flow is correct. According to Patterson and Hennessy's "Computer Organization and Design", TLB should be checked to obtain the physical address which contains physical address tag and cache index , and then you can access the cache based on the cache index and physical address tag.
Summet Singh Check out this figure in Patterson and Hennesey's book: harttle. The page fault occurs when the virtual address is not currently mapped to a physical address. Show 8 more comments. Tadele Ayelegn Tadele Ayelegn 3, 1 1 gold badge 29 29 silver badges 27 27 bronze badges. Just imagine a process is running and requires a data item X. Rupsingh Rupsingh 1, 1 1 gold badge 19 19 silver badges 31 31 bronze badges.
I am confused of the difference between TLB miss and page faults. Is page fault a crash? Or is it the same as a TLB miss? If the page is not in cache then it's a cache miss and further looks for the page in RAM.
TLB stores the physical address whereas Cache stores the word. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. In most cache hierarchies if you have an L1 cache miss, then that miss will probably be looked up in the L2. Doesn't matter if it is inclusive or not.
To do otherwise, you would have to have something that told you that the data you care about is probably not in the L2, you don't need to look. Although I have designed protocols and memory types that do this - e. Bit I am not aware of anyone shipping them at the moment.
Anyway, here are some reasons why the number of L1 cache misses may not be equal to the number of L2 cache accesses. You don't say what systems you are working on - I know my answer is applicable to Intel x86s such as Nehalem and Sandybridge, whose EMON performance event monitoring allows you to count things such as L1 and L2 cache misses, etc.
It will probably also apply to any modern microprocessor with hardware performance counters for cache misses, such as those on ARM and Power. Most modern microprocessors do not stop at the first cache miss, but keep going trying to do extra work.
This is overall often called speculative execution. Furthermore, the processor may be in-order or out-of-order, but although the latter may given you even greater differences between number of L1 misses and number of L2 accesses, it's not necessary - you can get this behavior even on in-order processors. Short answer: many of these speculative memory accesses will be to the same memory location. They will be squashed and combined. The performance event "L1 cache misses" is probably counting the number of speculative instructions that missed the L1 cache.
Which then allocate a hardware data structure, called at Intel a fill buffer, at some other places a miss status handling register.
Subsequent cache misses that are to the same cache line will miss the L1 cache but hit the fill buffer, and will get squashed. Only one of them, typically the first will get sent to the L2, and counted as an L2 access. But this may undercount, since speculation may pull the data into the cache, and a cache miss at retirement may never occur. Almost definitely. I might have to check the definition, look at the RTL, but I would be immensely surprised if not. It is almost guaranteed. If the address of A[0] is equal to zero modulo 64, then A[0]..
A[63] will be in the same cache line, on a machine with 64 byte cache lines. If the code that uses these is simple, it is quite possible that all of them can be issued speculatively. QED: 64 speculative memory access, 64 L1 cache misses, but only one L2 memory access.
By the way, don't expect the numbers to be quite so clean. You might not get exactly 64 L1 accesses per L2 access. If the number of L2 accesses is greater than the number of L1 cache misses I have almost never seen it, but it is possible you may have a memory access pattern that is confusing a hardware prefetcher.
The hardware prefetcher tries to predict which cache lines you are going to need. If the prefetcher predicts badly, it may fetch cache lines that you don't actually need.
Some machines may cancel speculative accesses that have caused an L1 cache miss, before they are sent to the L2. However, I don't know of Intel doing this. The pagemap is only applicable for virtual to physical address translation. However, as it's residing in memory and only partially cached in the TLBs, you may have to access it there during the translation process.
0コメント