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diff --git a/en_US.ISO8859-1/books/arch-handbook/vm/chapter.sgml b/en_US.ISO8859-1/books/arch-handbook/vm/chapter.sgml deleted file mode 100644 index 4710973d5f..0000000000 --- a/en_US.ISO8859-1/books/arch-handbook/vm/chapter.sgml +++ /dev/null @@ -1,255 +0,0 @@ -<!-- - The FreeBSD Documentation Project - - $FreeBSD: doc/en_US.ISO_8859-1/books/developers-handbook/usb/chapter.sgml,v 1.1 2001/04/13 09:05:13 murray Exp $ ---> - -<chapter id="vm"> - <title>Virtual Memory System</title> - - <sect1 id="internals-vm"> - <title>The FreeBSD VM System</title> - - <para><emphasis>Contributed by &a.dillon;. 6 Feb 1999</emphasis></para> - - <sect2> - <title>Management of physical - memory—<literal>vm_page_t</literal></title> - - <para>Physical memory is managed on a page-by-page basis through the - <literal>vm_page_t</literal> structure. Pages of physical memory are - categorized through the placement of their respective - <literal>vm_page_t</literal> structures on one of several paging - queues.</para> - - <para>A page can be in a wired, active, inactive, cache, or free state. - Except for the wired state, the page is typically placed in a doubly - link list queue representing the state that it is in. Wired pages - are not placed on any queue.</para> - - <para>FreeBSD implements a more involved paging queue for cached and - free pages in order to implement page coloring. Each of these states - involves multiple queues arranged according to the size of the - processor's L1 and L2 caches. When a new page needs to be allocated, - FreeBSD attempts to obtain one that is reasonably well aligned from - the point of view of the L1 and L2 caches relative to the VM object - the page is being allocated for.</para> - - <para>Additionally, a page may be held with a reference count or locked - with a busy count. The VM system also implements an <quote>ultimate - locked</quote> state for a page using the PG_BUSY bit in the page's - flags.</para> - - <para>In general terms, each of the paging queues operates in a LRU - fashion. A page is typically placed in a wired or active state - initially. When wired, the page is usually associated with a page - table somewhere. The VM system ages the page by scanning pages in a - more active paging queue (LRU) in order to move them to a less-active - paging queue. Pages that get moved into the cache are still - associated with a VM object but are candidates for immediate reuse. - Pages in the free queue are truly free. FreeBSD attempts to minimize - the number of pages in the free queue, but a certain minimum number of - truly free pages must be maintained in order to accommodate page - allocation at interrupt time.</para> - - <para>If a process attempts to access a page that does not exist in its - page table but does exist in one of the paging queues ( such as the - inactive or cache queues), a relatively inexpensive page reactivation - fault occurs which causes the page to be reactivated. If the page - does not exist in system memory at all, the process must block while - the page is brought in from disk.</para> - - <para>FreeBSD dynamically tunes its paging queues and attempts to - maintain reasonable ratios of pages in the various queues as well as - attempts to maintain a reasonable breakdown of clean v.s. dirty pages. - The amount of rebalancing that occurs depends on the system's memory - load. This rebalancing is implemented by the pageout daemon and - involves laundering dirty pages (syncing them with their backing - store), noticing when pages are activity referenced (resetting their - position in the LRU queues or moving them between queues), migrating - pages between queues when the queues are out of balance, and so forth. - FreeBSD's VM system is willing to take a reasonable number of - reactivation page faults to determine how active or how idle a page - actually is. This leads to better decisions being made as to when to - launder or swap-out a page.</para> - </sect2> - - <sect2> - <title>The unified buffer - cache—<literal>vm_object_t</literal></title> - - <para>FreeBSD implements the idea of a generic <quote>VM object</quote>. - VM objects can be associated with backing store of various - types—unbacked, swap-backed, physical device-backed, or - file-backed storage. Since the filesystem uses the same VM objects to - manage in-core data relating to files, the result is a unified buffer - cache.</para> - - <para>VM objects can be <emphasis>shadowed</emphasis>. That is, they - can be stacked on top of each other. For example, you might have a - swap-backed VM object stacked on top of a file-backed VM object in - order to implement a MAP_PRIVATE mmap()ing. This stacking is also - used to implement various sharing properties, including, - copy-on-write, for forked address spaces.</para> - - <para>It should be noted that a <literal>vm_page_t</literal> can only be - associated with one VM object at a time. The VM object shadowing - implements the perceived sharing of the same page across multiple - instances.</para> - </sect2> - - <sect2> - <title>Filesystem I/O—<literal>struct buf</literal></title> - - <para>vnode-backed VM objects, such as file-backed objects, generally - need to maintain their own clean/dirty info independent from the VM - system's idea of clean/dirty. For example, when the VM system decides - to synchronize a physical page to its backing store, the VM system - needs to mark the page clean before the page is actually written to - its backing s tore. Additionally, filesystems need to be able to map - portions of a file or file metadata into KVM in order to operate on - it.</para> - - <para>The entities used to manage this are known as filesystem buffers, - <literal>struct buf</literal>'s, and also known as - <literal>bp</literal>'s. When a filesystem needs to operate on a - portion of a VM object, it typically maps part of the object into a - struct buf and the maps the pages in the struct buf into KVM. In the - same manner, disk I/O is typically issued by mapping portions of - objects into buffer structures and then issuing the I/O on the buffer - structures. The underlying vm_page_t's are typically busied for the - duration of the I/O. Filesystem buffers also have their own notion of - being busy, which is useful to filesystem driver code which would - rather operate on filesystem buffers instead of hard VM pages.</para> - - <para>FreeBSD reserves a limited amount of KVM to hold mappings from - struct bufs, but it should be made clear that this KVM is used solely - to hold mappings and does not limit the ability to cache data. - Physical data caching is strictly a function of - <literal>vm_page_t</literal>'s, not filesystem buffers. However, - since filesystem buffers are used placehold I/O, they do inherently - limit the amount of concurrent I/O possible. As there are usually a - few thousand filesystem buffers available, this is not usually a - problem.</para> - </sect2> - - <sect2> - <title>Mapping Page Tables - vm_map_t, vm_entry_t</title> - - <para>FreeBSD separates the physical page table topology from the VM - system. All hard per-process page tables can be reconstructed on the - fly and are usually considered throwaway. Special page tables such as - those managing KVM are typically permanently preallocated. These page - tables are not throwaway.</para> - - <para>FreeBSD associates portions of vm_objects with address ranges in - virtual memory through <literal>vm_map_t</literal> and - <literal>vm_entry_t</literal> structures. Page tables are directly - synthesized from the - <literal>vm_map_t</literal>/<literal>vm_entry_t</literal>/ - <literal>vm_object_t</literal> hierarchy. Remember when I mentioned - that physical pages are only directly associated with a - <literal>vm_object</literal>. Well, that isn't quite true. - <literal>vm_page_t</literal>'s are also linked into page tables that - they are actively associated with. One <literal>vm_page_t</literal> - can be linked into several <emphasis>pmaps</emphasis>, as page tables - are called. However, the hierarchical association holds so all - references to the same page in the same object reference the same - <literal>vm_page_t</literal> and thus give us buffer cache unification - across the board.</para> - </sect2> - - <sect2> - <title>KVM Memory Mapping</title> - - <para>FreeBSD uses KVM to hold various kernel structures. The single - largest entity held in KVM is the filesystem buffer cache. That is, - mappings relating to <literal>struct buf</literal> entities.</para> - - <para>Unlike Linux, FreeBSD does NOT map all of physical memory into - KVM. This means that FreeBSD can handle memory configurations up to - 4G on 32 bit platforms. In fact, if the mmu were capable of it, - FreeBSD could theoretically handle memory configurations up to 8TB on - a 32 bit platform. However, since most 32 bit platforms are only - capable of mapping 4GB of ram, this is a moot point.</para> - - <para>KVM is managed through several mechanisms. The main mechanism - used to manage KVM is the <emphasis>zone allocator</emphasis>. The - zone allocator takes a chunk of KVM and splits it up into - constant-sized blocks of memory in order to allocate a specific type - of structure. You can use <command>vmstat -m</command> to get an - overview of current KVM utilization broken down by zone.</para> - </sect2> - - <sect2> - <title>Tuning the FreeBSD VM system</title> - - <para>A concerted effort has been made to make the FreeBSD kernel - dynamically tune itself. Typically you do not need to mess with - anything beyond the <literal>maxusers</literal> and - <literal>NMBCLUSTERS</literal> kernel config options. That is, kernel - compilation options specified in (typically) - <filename>/usr/src/sys/i386/conf/<replaceable>CONFIG_FILE</replaceable></filename>. - A description of all available kernel configuration options can be - found in <filename>/usr/src/sys/i386/conf/LINT</filename>.</para> - - <para>In a large system configuration you may wish to increase - <literal>maxusers</literal>. Values typically range from 10 to 128. - Note that raising <literal>maxusers</literal> too high can cause the - system to overflow available KVM resulting in unpredictable operation. - It is better to leave maxusers at some reasonable number and add other - options, such as <literal>NMBCLUSTERS</literal>, to increase specific - resources.</para> - - <para>If your system is going to use the network heavily, you may want - to increase <literal>NMBCLUSTERS</literal>. Typical values range from - 1024 to 4096.</para> - - <para>The <literal>NBUF</literal> parameter is also traditionally used - to scale the system. This parameter determines the amount of KVA the - system can use to map filesystem buffers for I/O. Note that this - parameter has nothing whatsoever to do with the unified buffer cache! - This parameter is dynamically tuned in 3.0-CURRENT and later kernels - and should generally not be adjusted manually. We recommend that you - <emphasis>not</emphasis> try to specify an <literal>NBUF</literal> - parameter. Let the system pick it. Too small a value can result in - extremely inefficient filesystem operation while too large a value can - starve the page queues by causing too many pages to become wired - down.</para> - - <para>By default, FreeBSD kernels are not optimized. You can set - debugging and optimization flags with the - <literal>makeoptions</literal> directive in the kernel configuration. - Note that you should not use <option>-g</option> unless you can - accommodate the large (typically 7 MB+) kernels that result.</para> - - <programlisting>makeoptions DEBUG="-g" -makeoptions COPTFLAGS="-O -pipe"</programlisting> - - <para>Sysctl provides a way to tune kernel parameters at run-time. You - typically do not need to mess with any of the sysctl variables, - especially the VM related ones.</para> - - <para>Run time VM and system tuning is relatively straightforward. - First, use softupdates on your UFS/FFS filesystems whenever possible. - <filename>/usr/src/contrib/sys/softupdates/README</filename> contains - instructions (and restrictions) on how to configure it up.</para> - - <para>Second, configure sufficient swap. You should have a swap - partition configured on each physical disk, up to four, even on your - <quote>work</quote> disks. You should have at least 2x the swap space - as you have main memory, and possibly even more if you do not have a - lot of memory. You should also size your swap partition based on the - maximum memory configuration you ever intend to put on the machine so - you do not have to repartition your disks later on. If you want to be - able to accommodate a crash dump, your first swap partition must be at - least as large as main memory and <filename>/var/crash</filename> must - have sufficient free space to hold the dump.</para> - - <para>NFS-based swap is perfectly acceptable on -4.x or later systems, - but you must be aware that the NFS server will take the brunt of the - paging load.</para> - </sect2> - </sect1> - -</chapter> |