Windows® Internals, Sixth Edition, Part 1

To begin with, the session ID is stored, and the CPU share weight is set to its default of 5. You’ll see shortly what a weight is, how it can be computed, and its effects on the DFSS engine. Because the quota block has just been created, the initial cycle values are all set to their maximum value for now. Next, this new per-session CPU block must be visible to the system. Therefore, the PspCpuQuotaControl data structure is updated with the new total weight of all sessions (by adding this weight), and the quota block is inserted into the block list (sorted by session ID). Finally, PspCalculateCpuQuotaBlockCycleCredits enumerates every other session’s quota block and captures the new total weight of the system.

Once this is done, the per-session CPU quota block is finalized, and the memory manager sets it in the CpuQuotaBlock field of the MM_SESSION_SPACE structure for this session. Likewise, the current EPROCESS (part of this new session’s CpuQuotaBlock field) is also updated to point to this session’s CPU quota block. Now that the process has received a CPU quota block as soon as it became part of the session, future threads created by this process (including the first thread itself) will be allocated with an extra structure after their typical ETHREAD—a per-process CPU Quota APC structure. Additionally, the ETHREAD’s RateApcState field will be set to PsRateApcContained, indicating that this is an embedded Quota APC, as used by the DFSS mechanism (rather than the pool-allocated legacy APC). Finally, the CpuThrottled bit is set in the KTHREAD’s ThreadControlFlags.

At this point, the global quota-control structure contains a pointer to the DFSS per-CPU data structure array, which itself is linked to all the per-session CPU blocks that have been created for each session and associated with the EPROCESS structure of the member processes. In turn, each thread part of such a process has CPU throttling turned on. There is a per-CPU DPC ready to execute, as well as per-thread APCs for each throttled thread.

When the last process in the session loses all its references, PsDeleteCpuQuotaBlock is called. It removes the block from the list, refreshes the total weights, and calls PspCalculateCpuQuotaBlockCycleCredits to update all other per-session CPU quota blocks.

Charging of Cycles to Throttled Threads

After everything is set up, the entire DFSS mechanism is triggered by the consumption of CPU cycles—something that was already explained in the earlier sections. In other words, not only are consumed cycles used for quantum accounting and providing finer-grained information to thread APIs, but they also can be “charged” against the thread (and thus against its quota). This operation is done by the PsChargeProcessCpuCycles function that is called whenever a thread has completed the accumulation of cycles in its current execution timeline.

The first operation involves accumulating the additional cycles to the per-CPU DFSS data structure for this processor, increasing the TotalCyclesAccumulated value. If this accumulation has reached the total credit, the quota DPC is immediately queued. Once the DPC ultimately executes, it calls PspStartNewFairShareInterval, which updates the generation, resets the cycles accumulated, and resets the credit to 150 ms. Finally, the idle-only queue is flushed on each processor associated with a given session. (You’ll see what this queue is and what flushing it entails, later.) This part of the algorithm manages the 150-ms interval that controls DFSS.