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Lay out data structures efficiently and resue them when possible
1. Design issue
Plan your memory allocations according to type of memory, size, and lifetime.
Combine allocations of similar lifetimes, so that you can free unused memory as soon as it is no longer needed.
Don’t mix structures of greatly different sizes in the same allocation unless you can be sure that they will be aligned appropriately.
2. Reusability
Reuse structures instead of freeing them and later reallocating memory for other uses.
3. I/O request handling
Drivers often require additional memory while handling I/O requests.
The ways could be
- Allocate a memory descriptor list (MDL) or internal buffer to use for a specific I/O request
- Allocate an IRP to send to lower drivers.
The size of these structures varies depending on the request.
If your driver has a technique to limit I/O size or to split up a large I/O request, you could make the buffer a fixed size
4. Optimize for the most frequent operations
Allocate nonpaged pool memory for long-term use at start up.
1. Allocate memory for long-term use in a
DriverEntry or
AddDevice routine and free the memory as part of handling the device removal request.
2. The driver should not, however, preallocate excessively large blocks of memory (several megabytes, for example)
3. Appropriate memory allocation routines
- ExAllocatePoolWithTag
- ExAllocatePoolWithQuotaTag
- ExAllocatePoolWithTagPriority
- AllocateCommonBuffer (if the driver's device uses bus-master DMA or a system DMA controller's auto-initialize mode).
4. Use the tagged versions of the pool allocation routines instead of the nontagged versions, this will be helpful at debug and tracking.
Use memory economically.
Nonpaged pool memory is a limited system resource.
1. General Rules:
Avoid calling the memory allocation support routines repeatedly to request allocations of less than PAGE_SIZE.
Bundling structures into a single allocation,if your driver normally uses these related structures together.
2. Exception: (Drivers that use DMA)
If a driver that performs DMA needs several one-page buffers, but the buffers need not be contiguous, it should call
AllocateCommonBuffer once for each such buffer.
3. Round the allocation request up to the next page boundary.
ExAllocatePoolWithTag
- ＜PAGE_SIZE: allocates the number of bytes requested, they are aligned on an 8-byte boundary.
- >=PAGE_SIZE: allocates a page-aligned buffer that is an integral multiple of PAGE_SIZE bytes
AllocateCommonBuffer always allocates at least a page of memory. If the driver requests less than an integral multiple of PAGE_SIZE bytes, the remaining bytes on the last page are inaccessible to the driver.
Use lookaside lists.
1. Lookaside lists provide fixed-size, reusable buffers.
2. They are designed for structures that a driver might need to allocate dynamically and in unpredictable numbers.
3. Lookaside lists can be allocated from paged pool or nonpaged pool.
4. The driver defines the layout and contents of the entries in the list to suit its requirements, and the system maintains list status and adjusts the number of available entries according to demand.
- ExInitialize[N]PagedLookasideList to set up a lookaside list
- ExAllocateFrom[N]PagedLookasideList to allocate an entry in the list
- ExFreeTo[N]PagedLookasideList to free an entry in the list. The head of the list must be allocated from nonpaged memory, even if the list entries themselves are in paged memory.
Avoid frequently mapping and unmapping the virtual memory address space
This kind of operations can
decrease performance system-wide
Because it can result in frequent flushes of the translation lookaside buffer (TLB), a per-processor cache of virtual-to-physical address translations. Each entry in the TLB contains a page table entry (PTE).
Use
direct I/O to avoid this problem.
Internally, the I/O manager avoids this problem for the MDL in Irp->MdlAddress . The first time a kernel-mode component calls MmGetSystemAddressForMdlSafe , the I/O manager stores the system address in the MDL along with the corresponding physical address. When the IRP returns to the I/O manager after completion,, the I/O manager unmaps the MDL. Thus, the I/O manager requires only a single mapping (and a single virtual to physical address translation) for each I/O request.
Test and verify
Use Driver Verifier (verifier.exe), GFlags (gflags.exe), and PoolMon (poolmon.exe) to track, test, and verify memory allocation issues.
Driver verifier can allocate memory from a special pool and monitor the driver’s access to the allocated memory. It can detect attempts to access memory outside the allocated range or after the memory has been freed. In addition, it checks for memory leaks—memory that has been allocated but is no longer being used and is never freed. Verifier also gathers statistics on the number of memory allocations requested from the special pool and whether they succeeded or failed.
GFlags works together with Driver Verifier. Using GFlags, you can configure the Special Memory Pool option of Driver Verifier or designate the special pool for use in individual memory allocations.
PoolMon gathers and displays a variety of data about memory allocation, sorted by the pool tags assigned during allocation.
還記得Google Map剛出來的時候，台灣的地圖總是一片空白，到了現在總算出現街道圖了，還挺漂亮的呢。這下子Urmap有點危險囉，地圖沒有Google Map好看，而且Google Map上面還有附上每條路(單行道)的方向，就看看它能不能拿出更多在地化的服務吧，不然...
為什麼會有這麼多專家學者，替這樣的言論背書呢？可以看看Exxon Secrets這個網站，他認為這是因為Exxon Mobil(美孚石油)他們提供大量的贊助金額給這些研究機構，去提出有利於他們的言論，以避免受到社會指責，並且繼續行銷他們的商品 - 石油。(假如不想看Flash，這有篇關於這個網頁的報導)
Free Programming Resources
FreeTechBooks.com
Computer-Books.us
OOPWeb.com
今天，提供一個比較另類的避免垃圾郵件的方法，那就是使用可丟棄式的垃圾郵件。
幸福是什麼??
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4月22日，世界地球日。
但是日本和德國卻可以一直維持在10以下，這其中有大的原因是因為我們的發電來源，仍有大部分是火力發電的部份，而且是用燃煤，不僅效率差，對空氣環境的影響也是相對的大很多。另外，氣候也是另一個影響因素，天氣太熱或太冷，那麼空調的使用就會造成更多能源的耗費，也就可能會造成更多的CO2，不過綠建築的建造，可以減少這方面的影響。
人的一生，有時候跟你在過的每一天極微類似。
每個人都知道很多很多有多少人真的因此而少開車，多用大眾運輸系統呢？
kDice則是搶地盤的遊戲，骰子就是你的士兵。規則如下:
