Windows® Internals, Sixth Edition, Part 2

You can see the cache manager map views into the system cache and flush pages to disk by running the Performance Monitor and adding the Data Maps/sec and Lazy Write Flushes/sec counters and then copying a large file from one location to another. The generally higher line in the following screen shot shows Data Maps/sec and the other shows Lazy Write Flushes/sec. During the file copy, Lazy Write Flushes/sec significantly increased.

Write Throttling

The file system and cache manager must determine whether a cached write request will affect system performance and then schedule any delayed writes. First the file system asks the cache manager whether a certain number of bytes can be written right now without hurting performance by using the CcCanIWrite function and blocking that write if necessary. For asynchronous I/O, the file system sets up a callback with the cache manager for automatically writing the bytes when writes are again permitted by calling CcDeferWrite. Otherwise, it just blocks and waits on CcCanIWrite to continue. Once it’s notified of an impending write operation, the cache manager determines how many dirty pages are in the cache and how much physical memory is available. If few physical pages are free, the cache manager momentarily blocks the file system thread that’s requesting to write data to the cache. The cache manager’s lazy writer flushes some of the dirty pages to disk and then allows the blocked file system thread to continue. This write throttling prevents system performance from degrading because of a lack of memory when a file system or network server issues a large write operation.

Note

The effects of write throttling are volume-aware, such that if a user is copying a large file on, say, a RAID-0 SSD while also transferring a document to a portable USB thumb drive, writes to the USB disk will not cause write throttling to occur on the SSD transfer.

The dirty page threshold is the number of pages that the system cache will allow to be dirty before throttling cached writers. This value is computed at system initialization time and depends on the product type (client or server). Two other values are also computed—the top dirty page threshold and the bottom dirty page threshold. Depending on memory consumption and the rate at which dirty pages are being processed, the lazy writer calls the internal function CcAdjustThrottle, which, on server systems, performs dynamic adjustment of the current threshold based on the calculated top and bottom values. This adjustment is made to preserve the read cache in cases of a heavy write load that will inevitably overrun the cache and become throttled. Table 11-1 lists the algorithms used to calculate the dirty page thresholds.

Table 11-1. Algorithms for Calculating the Dirty Page Thresholds

Product Type

Dirty Page Threshold

Top Dirty Page Threshold

Bottom Dirty Page Threshold

Client

Physical pages / 8

Physical pages / 8

Physical pages / 8

Server

Physical pages / 2

Physical pages / 2

Physical pages / 8

Write throttling is also useful for network redirectors transmitting data over slow communication lines. For example, suppose a local process writes a large amount of data to a remote file system over a 9600-baud line. The data isn’t written to the remote disk until the cache manager’s lazy writer flushes the cache. If the redirector has accumulated lots of dirty pages that are flushed to disk at once, the recipient could receive a network timeout before the data transfer completes. By using the CcSetDirtyPageThreshold function, the cache manager allows network redirectors to set a limit on the number of dirty cache pages they can tolerate (for each stream), thus preventing this scenario. By limiting the number of dirty pages, the redirector ensures that a cache flush operation won’t cause a network timeout.

EXPERIMENT: Viewing the Write-Throttle Parameters

The !defwrites kernel debugger command dumps the values of the kernel variables the cache manager uses, including the number of dirty pages in the file cache (CcTotalDirtyPages), when determining whether it should throttle write operations:lkd> !defwrites *** Cache Write Throttle Analysis *** CcTotalDirtyPages: 39 ( 156 Kb) CcDirtyPageThreshold: 32753 ( 131012 Kb) MmAvailablePages: 81569 ( 326276 Kb) MmThrottleTop: 450 ( 1800 Kb) MmThrottleBottom: 80 ( 320 Kb) MmModifiedPageListHead.Total: 4337 ( 17348 Kb) Write throttles not engaged