TLB and Pagewalk Coherence in x86 Processors

Processors that support paging use TLBs to cache translations. On x86, translation caches are not coherent and requires software to explicitly invalidate a TLB entry after updating a page table entry. Similarly, pagewalks are not guaranteed to be coherent, so modifying a page table entry must be followed by an invalidation even if the page table entry is not cached in the TLB.

Real processor implementations do not provide TLB coherence, but it turns out many (but not all) processors actually do provide pagewalk coherence. Most provide pagewalk coherence by detecting when page table entry update conflicts with a pagewalk’s memory accesses, but some provide coherence by disallowing speculative pagewalks, at some performance cost. I show a microbenchmark that can test for TLB and pagewalk coherence and whether speculative pagewalks are used.

. . . → Read More: TLB and Pagewalk Coherence in x86 Processors

Windows 9x TLB Invalidation Bug

In processor architectures that support paging, there are usually one or more TLBs or pagewalk caches to cache address translations. On x86, these translation caches are not coherent with memory accesses that modify the page tables, and need invalidating after a page table entry is modified.

The Windows 9x kernel contains code that modifies a page table entry, then immediately uses it without an invalidation. This causes crashes if the processor strictly follows the instruction set specification and does not provide pagewalk coherence.

. . . → Read More: Windows 9x TLB Invalidation Bug

ASRock H81M-ITX Overclocking

The ASRock H81M-ITX does support adjusting multiplier ratios for K series Haswell processors. Oddly, this works in BIOS version 1.90, but not version 2.00. My boards came with version 2.00, and I had to downgrade to 1.90.

The four-phase VRM does make a small amount of noise and gets rather hot. There are no heatsinks . . . → Read More: ASRock H81M-ITX Overclocking

Bash bug patch for older Mandriva distros

Bash has bugs. Unfortunately, the bash parser was exposed through environment function importing, which had the potential for remotely exploiting the parser bugs. There’s been a series of patches for these issues. I’ve compiled packages of bash 4.3 for older versions of Mandriva Linux. . . . → Read More: Bash bug patch for older Mandriva distros

RTL8192CU and Linux 3.13.10

Hardware: TP-Link TL-WN823N v1.1

Status: As of kernel 3.13.10, the in-tree rtl8192cu driver (in drivers/net/wireless/rtlwifi) is still broken. It will work and connect, but will silently disconnect after some time (and light traffic?). There is also some packet loss (around 1%). The driver provided by Realtek (8192cu) works much better. . . . → Read More: RTL8192CU and Linux 3.13.10

Store-to-Load Forwarding and Memory Disambiguation in x86 Processors

In pipelined processors, instruction are executed speculatively and are not permitted to modify system state until instruction commit. For stores to memory, speculative stores write into a store queue at execution time and only write into cache after the store instructions have committed. Out of order memory execution requires hardware that learns dependencies between stores and loads, and also the ability to forward stored values from the store queue to loads that depend on them. I describe two variations of a microbenchmark that can measure some aspects of store-to-load forwarding and the memory execution hardware. These showed that AMD’s Bulldozer and Piledriver processors likely do not use a dynamic memory dependence predictor. They were also used to generate interesting 2D charts that can reveal some details about how the memory execution hardware might be designed. . . . → Read More: Store-to-Load Forwarding and Memory Disambiguation in x86 Processors

AMD Bulldozer/Piledriver Modules and Hyper-Threading

Ever since Intel’s Hyper-Threading and AMD’s Bulldozer modules, there has been much debate on what qualifies as a real CPU “core”. Unfortunately, I don’t think “core” is easy to define, so marketing tends to name things for their own benefit. In the end, it’s the performance that matters, not the name. Two-way Hyper-Threading gives around 23% improvement over one thread, while two-way multithreading in a “module” gives 54%. This is still quite far from >90% that replicating the entire CPU core would achieve . . . → Read More: AMD Bulldozer/Piledriver Modules and Hyper-Threading

More Cardboard Boxes

After having built small cardboard cases for single computers, I tried building single cases for multiple machines. The idea is to share the power supply between four systems to reduce cost and increase packing density. Here are some pictures of two such systems I built . . . → Read More: More Cardboard Boxes

Measuring Reorder Buffer Capacity

On conventional out of order processors, instructions are not necessarily executed in “program order”, although the processor must give the same results as though execution occurred in program order. The instruction window contains a small window of instructions that are allowed to execute out of order, before the instructions are committed in program order as they leave the instruction window. This article describes a microbenchmark that can measure the size of the instruction window, demonstrated on several x86 microarchitectures, then extends the microbenchmark to measure the speculative register file size. . . . → Read More: Measuring Reorder Buffer Capacity

Intel Ivy Bridge Cache Replacement Policy

Caches are used to store a subset of a larger memory space in a smaller, faster memory, with the hope that future memory accesses will find their data in the cache. Traditionally, caches have used (approximations of) the least-recently used (LRU) replacement policy, but LRU performs poorly with cyclic access patterns with working sets larger than the cache. Intel Ivy Bridge’s L3 cache uses an improved adaptive replacement policy, and is no longer purely pseudo-LRU . . . → Read More: Intel Ivy Bridge Cache Replacement Policy