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FreeBSD VM overview (freebsd vm memory proccess)


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_ RU.OS.CMP (2:5077/15.22) _________________________________________ RU.OS.CMP _ From : Vadim Kolontsov 2:5020/400 17 Jan 99 22:34:18 Subj : FreeBSD VM overview ________________________________________________________________________________ From: nospam.vadim@tversu.ru (Vadim Kolontsov) Reply-To: nospam.vadim@tversu.ru Привет, для более аргументированного сравнения... приношу свои извинения, если этот текст появлялся здесь неоднократно. Оригинал - на сайте автора, http://apollo.backplane.com (там же - комментарии на тему предстоящих изменений VM в FreeBSD-4.0) В следующем письме -- Linux VM system overview. А есть подобный документ про Windows-NT и Windows-95? Было бы, наверное, очень интересно почитать. V. ----------------------------------------------------------------------------- FREEBSD VM SYSTEM OVERVIEW By Matthew Dillon, with additional notes from the creator, John Dyson. Paragraphs marked (note 1) are annotations made by John Dyson to my original document. The document is meant to describe the general workings of FreeBSD's VM system to interested parties. VM BUFFER CACHE Lets see... ok. At its lowest level the VM system consists of nothing more then the buffer cache. This cache contains every single page of physical memory. Each page of physical memory has a vm_page_t structure associated with it. Each page is indexed by (object, page-index). So, for example, an object might represent a buffered disk device and the index would then represent a page within that device. The buffer cache maintains a hashtable allowing the system to locate any piece of the object in the buffer cache. Being a cache, whole objects are not necessarily stored so if the system is unable to locate a particular page, it needs to allocate a page from the free pool and then initiate the appropriate I/O operation to load it. Each page is placed in one of several buckets depending on its state: active pages actively used by programs. inactive pages not actively used by programs which are dirty and (at some point) need to be written to their backing store (typically disk). These pages are still associated with objects and can be reclaimed if a program references them. Pages can be moved from the active to the inactive queue at any time with little adverse effect. Moving pages to the cache queue has bigger consequences (note 1) cache pages not actively used by programs which are clean and can be thrown away (moved to the free bucket) at any time. These pages are still associated with objects and can be reclaimed if a program references them. The cache pages are available only at non-interrupt time. (note 1) free pages not used by anyone. These pages are not associated with objects. A limited number of free pages are kept in reserve at all times. Older version of BSD had to keep a larger number of free pages to perform correctly, and now the cache queue helps with that purpose. (note 1). FreeBSD will use 'all of memory' for the disk cache. What this means is that the 'free' bucket typically contains only a few pages in it. If the system runs out, it can free up more pages from the cache bucket. System activity works like this: When a program actively references a page in a file on the disk (etc...) the page is brought into the buffer cache via a physical I/O operation. It typically goes into the 'active' bucket. If a program stops referencing the page, the page slowly migrates down into the inactive or cache buckets (depending on whether it is dirty or not). Dirty pages are slowly 'cleaned' by writing them to their backing store and moved from inactive to cache, and cache pages are freed as necessary to maintain a minimum number of truely free pages in the free bucket. These pages can still be 'cleaned' by allocating swap as their backing store, allowing them to migrate through the buckets and eventually be reused. On the flip side, programs are continually allocating and freeing memory. Memory not associated with backing store is allocated out of the free list and freed directly back to the freelist. If the system eats the free list too much, it starts to pull pages out of the cache and put them into the free list. This, in turn, may starve the cache bucket and cause the system to work harder to clean pages from the inactive bucket so it can move them into the cache, and to deactivate active pages so it can move them into the inactive or cache buckets. On a very heavily loaded system, the migration of pages between buckets goes faster and faster and results in more disk I/O as inactive pages are cleaned (written to swap or disk) and as cache pages are thrown away and then later rereferenced by some program, causing a physical I/O to occur. One of FreeBSD's greatest strengths is it's ability to dynamically tune itself to the load situation on the machine by dynamically adjusting 'target numbers' for the various buckets and then moving pages between the buckets (with the side effect of causing paging, swapping, and disk I/O to occur) to meet the target numbers. The tuning is partially due to locking the scan-rate to "demand" on memory instead of arbitrary time. The traditional arbitrary real time scanning distorts the stats gathering, and is a fatal flaw under load. It is also importatant to limit the scan rate. So, the domain for the FreeBSD memory management code is time and recent usage, rather than bogus memory address or just lru position on the queue. (The usage of the word "domain" above, is the mathematical defn, rather than common usage.) (note 1). The VM buffer cache caches everything the underlying storage so, for example, it will not only cache the data blocks associated with a file but it will cache the inode blocks and bitmap blocks as well. Most filesystem operations thus go very fast even for tripple-indirect block lookups and such. BUFFER POINTERS FreeBSD also has another buffer cache, called the 'filesystem buffer cache'. This cache is really just an indirect pointer to the VM buffer cache.. no actual copying is done in most cases. While treated as a separate cache, both the filesystem and VM buffer caches reference the same underlying pages and so you get the term 'unified buffer cache'. Mostly, the "buffer cache" is a temporary wired mapping scheme for I/O requests. This allows for compatibility with legacy interfaces, with hopefully minor additional overhead. (Of course, IMO, that additional overhead is a little too high for my taste.) (note 1) The filesystem buffer cache is responsible for collecting random pages from the VM buffer cache into larger contiguous pieces for the filesystem code to mess around with. For example, the system page size could be 4K but the filesystem block size might be 8K. The filesystem buffer cache remaps the pages from the VM buffer cache into KVM (kernel virtual memory) via the MMU and is able to thus present 'contiguous' areas of data to the filesystem code. This same mechanism is used to aid in 'clustering' pages together in order to do more efficient larger I/O's. For example, the hardware page size is 4K but swap operations are typically grouped in blocks of 16K, and filesystem operations in blocks of 8K. SWAP Swap space is used to assign backing store to 'unbacked memory'. For example, memory a program malloc()'s. The memory that a program uses is typically a combination of file-backed and unbacked memory. For example, when a program's CODE is loaded into memory it is typically simply mapped directly from the program file. If any of these pages wind up being unused, the system can simply throw them away (and reload them later if necessary). A program's DATA area is also mapped into memory, but in a manner that allows the program to modify the data. If the program modifies the file-backed DATA area the pages in question are reassign to 'unbacked memory' (so modifications to the program's data area are not improperly written back to the program file on disk!). The system faces a problem (which swap solves) for unbacked pages... it can't throw them away because these pages are typically dirty (i.e. modified). In order to reuse unbacked idle memory for other purposes, the system needs to be able to write these pages somewhere before it can throw them out of physical memory. This is where SWAP comes in. UNIXes usually come in two flavors when it comes to assigning SWAP. Most SYS-V based systems pre-allocate swap. Each page of unassociated memory is assign a swap block even if it is never written to swap. I think Solaris still does this. IRIX did until around 6.3 or so. Also, some UNIX's are really dumb about assigning swap. If you have a shared memory segment, some UNIX's will preallocate swap for the memory segment times each process that is sharing it, which is very wasteful. This is because these UNIX's assign swap based on the process's VM space map rather then based on the pages making up the VM space map and then go through hoops to 'fix it up' for memory segments that are truely shared. FreeBSD has arguably some of the best swap code in existance. I personally like it better then Linux's. Linux is lighter on swap, but doesn't balance system memory resource utilization well under varying load conditions. FreeBSD does. FreeBSD notes the uselessness of existing pages in memory, and decides that it might be advantageous to free memory (enabled by pushing pages to swap), so that it can be used for more active purposes (such as file buffering, or more program space.) It is a terrible waste to keep unused pages around, for the notion of saving (cheap) disk space. Since low level SWAP I/O can be faster, with less CPU overhead than file I/O, it is likely desireable to push such unused pages out so that they can be freed for use by higher overhead mechanisms. (note 1) In anycase, FreeBSD only allocates swap backing store to unbacked pages of memory when it decides it actually wants to clean those pages. It typically allocates swap blocks in 16K chunks. Once allocated, the pages in question are then written to swap space on the disk and moved from the inactive bucket to the cache bucket. From there they may be reclaimed by the program or moved into the free bucket when the system runs out of free memory and reused for other purposes. If after being cleaned the page is modified again, the page then moves back to the 'active' or 'inactive' buckets and the underlying swap space, now invalid, is deallocated. If the page is brought back into memory from swap and not modified, the swap space is typically left allocated to allow the page to be thrown away again without having to re-write it to disk. When a page is swapped out and reused, FreeBSD must maintain the swap reference information for that page somewhere (i.e. 'index X in some object O exists in swap block B'). This information is attached to the VM object representing the area of memory in question and 'compressed' by collapsing contiguous regions of allocated swap together. I don't quite recall whether FreeBSD allocates swap the way 4.3BSD did, but if so what it does is try to allocate a larger contiguous region of swap (i.e. 16K, 32K, 64K, etc...) and then assign contiguous pages in a VM object (such as a program's RSS and DATA areas) to contiguous pages of swap, allowing the reference information for that chunk to represent a considerable amount of swapped out memory. Thus FreeBSD will optimally manage the swap for the system no matter whether you have only a little swap (like a hundred megabytes) or a lot of swap (like a few gigabytes). VNODE CACHE, INODE CACHE, NAMEI CACHE As with most UNIX's, FreeBSD also maintains a cache for higher level 'raw' objects. A VNODE/INODE typically represents a file, whereas a NAMEI cache object represents a directory entry. So, for example, if you open() a file, write to it, and then close() it, FreeBSD will remember the name->inode association for the file and even cache most of the information so the next time you open() it, FreeBSD will be able to run the open nearly instantaniously using the cached information. This reduces the amount of directory searching and unnecessary extra file I/O required to operate on a file. ----------------------------------------------------------------------------- --- ifmail v.2.14dev2 * Origin: Tver State University NOC (2:5020/400@fidonet)

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