8 MB L3 Cache And Hyper-Threading
256 KB L2-Cache and 8 MB L3 Cache
Of the many changes that Intel has made with Nehalem, the new cache structure is among the most pivotal in improving performance compared to the Core 2. The L1 cache remains unchanged, divided into 32 KB data and 32 KB instruction parts. However, where the Core 2 Duo processor sports a shared cache of up to 6 MB for its two cores, each of the four cores of the Core i7 gets its own 256 KB L2 cache. Additionally, the processor comes with 8 MB of L3 cache that is shared by all four cores.
The advantage of this configuration is that a single-threaded application has access to the full 8 MB of L3 cache. This was not possible on the Core 2 Quad processors, since their 12 MB L2 cache consisted of two halves, each located on one of the dual-core dies that made up the quad-core package.
Another advantage of the large L3 cache is that all four cores can work with a single set of data, rather than having to duplicate it across several caches; this saves space and allows more data to be kept in the cache. Exchanging data between cores also benefits from a major speed increase compared to the Core 2 CPUs with two separate caches.
Hyper-Threading was originally introduced in the Pentium 4 / Pentium D line with its NetBurst architecture, but failed to provide any meaningful performance improvements. While it had disappeared from the desktop—the last CPU to use it was the Pentium Extreme Edition 965—it has surfaced in other chips such as the Atom.
The original intent was to improve multitasking capabilities in the desktop segment. To some extent, this worked out quite well, and was quite noticeable in everyday scenarios. In the case of current quad-core processors, Hyper-Threading has much less of an effect on their multi-threading capabilities, since they already offer enough cores to handle a large number of threads. Nonetheless, since modern applications run several threads simultaneously, they can utilize the additional virtual cores, making better use of the CPU overall.
When it introduced its Core 2 processors, Intel seemed to be saying its farewell to Hyper-Threading technology, but it has returned with the Core i7: all Core i7 models are equipped with Hyper-Threading. During our tests, only a few current applications saw marginal performance penalties. Overall, the numbers seem to confirm that the reintroduction of Hyper-Threading was a solid decision on Intel’s part. We were surprised by how many applications had a positive reaction to Hyper-Threading, showing performance gains.