Documentation for /proc/sys/vm/* kernel version 2.2.10 (c) 1998, 1999, Rik van Riel For general info and legal blurb, please look in README. ============================================================== This file contains the documentation for the sysctl files in /proc/sys/vm and is valid for Linux kernel version 2.2. The files in this directory can be used to tune the operation of the virtual memory (VM) subsystem of the Linux kernel, and one of the files (bdflush) also has a little influence on disk usage. Default values and initialization routines for most of these files can be found in mm/swap.c. Currently, these files are in /proc/sys/vm: - bdflush - buffermem - freepages - kswapd - overcommit_memory - page-cluster - pagecache - pagetable_cache ============================================================== bdflush: This file controls the operation of the bdflush kernel daemon. The source code to this struct can be found in linux/mm/buffer.c. It currently contains 9 integer values, of which 6 are actually used by the kernel. From linux/fs/buffer.c: -------------------------------------------------------------- union bdflush_param{ struct { int nfract; /* Percentage of buffer cache dirty to activate bdflush */ int ndirty; /* Maximum number of dirty blocks to write out per wake-cycle */ int nrefill; /* Number of clean buffers to try to obtain each time we call refill */ int nref_dirt; /* Dirty buffer threshold for activating bdflush when trying to refill buffers. */ int dummy1; /* unused */ int age_buffer; /* Time for normal buffer to age before we flush it */ int age_super; /* Time for superblock to age before we flush it */ int dummy2; /* unused */ int dummy3; /* unused */ } b_un; unsigned int data[N_PARAM]; } bdf_prm = {{40, 500, 64, 256, 15, 30*HZ, 5*HZ, 1884, 2}}; -------------------------------------------------------------- The first parameter governs the maximum number of dirty buffers in the buffer cache. Dirty means that the contents of the buffer still have to be written to disk (as opposed to a clean buffer, which can just be forgotten about). Setting this to a high value means that Linux can delay disk writes for a long time, but it also means that it will have to do a lot of I/O at once when memory becomes short. A low value will spread out disk I/O more evenly. The second parameter (ndirty) gives the maximum number of dirty buffers that bdflush can write to the disk in one time. A high value will mean delayed, bursty I/O, while a small value can lead to memory shortage when bdflush isn't woken up often enough... The third parameter (nrefill) is the number of buffers that bdflush will add to the list of free buffers when refill_freelist() is called. It is necessary to allocate free buffers beforehand, since the buffers often are of a different size than memory pages and some bookkeeping needs to be done beforehand. The higher the number, the more memory will be wasted and the less often refill_freelist() will need to run. When refill_freelist() comes across more than nref_dirt dirty buffers, it will wake up bdflush. Finally, the age_buffer and age_super parameters govern the maximum time Linux waits before writing out a dirty buffer to disk. The value is expressed in jiffies (clockticks), the number of jiffies per second is 100, except on Alpha machines (1024). Age_buffer is the maximum age for data blocks, while age_super is for filesystem metadata. ============================================================== buffermem: The three values in this file correspond to the values in the struct buffer_mem. It controls how much memory should be used for buffer memory. The percentage is calculated as a percentage of total system memory. The values are: min_percent -- this is the minimum percentage of memory that should be spent on buffer memory borrow_percent -- UNUSED max_percent -- UNUSED ============================================================== freepages: This file contains the values in the struct freepages. That struct contains three members: min, low and high. The meaning of the numbers is: freepages.min When the number of free pages in the system reaches this number, only the kernel can allocate more memory. freepages.low If the number of free pages gets below this point, the kernel starts swapping agressively. freepages.high The kernel tries to keep up to this amount of memory free; if memory comes below this point, the kernel gently starts swapping in the hopes that it never has to do real agressive swapping. ============================================================== kswapd: Kswapd is the kernel swapout daemon. That is, kswapd is that piece of the kernel that frees memory when it gets fragmented or full. Since every system is different, you'll probably want some control over this piece of the system. The numbers in this page correspond to the numbers in the struct pager_daemon {tries_base, tries_min, swap_cluster }; The tries_base and swap_cluster probably have the largest influence on system performance. tries_base The maximum number of pages kswapd tries to free in one round is calculated from this number. Usually this number will be divided by 4 or 8 (see mm/vmscan.c), so it isn't as big as it looks. When you need to increase the bandwidth to/from swap, you'll want to increase this number. tries_min This is the minimum number of times kswapd tries to free a page each time it is called. Basically it's just there to make sure that kswapd frees some pages even when it's being called with minimum priority. swap_cluster This is the number of pages kswapd writes in one turn. You want this large so that kswapd does it's I/O in large chunks and the disk doesn't have to seek often, but you don't want it to be too large since that would flood the request queue. ============================================================== overcommit_memory: This value contains a flag that enables memory overcommitment. When this flag is 0, the kernel checks before each malloc() to see if there's enough memory left. If the flag is nonzero, the system pretends there's always enough memory. This feature can be very useful because there are a lot of programs that malloc() huge amounts of memory "just-in-case" and don't much of it. Look at: mm/mmap.c::vm_enough_memory() for more information. ============================================================== page-cluster: The Linux VM subsystem avoids excessive disk seeks by reading multiple pages on a page fault. The number of pages it reads is dependent on the amount of memory in your machine. The number of pages the kernel reads in at once is equal to 2 ^ page-cluster. Values above 2 ^ 5 don't make much sense for swap because we only cluster swap data in 32-page groups. ============================================================== pagecache: This file does exactly the same as buffermem, only this file controls the struct page_cache, and thus controls the amount of memory used for the page cache. In 2.2, the page cache is used for 3 main purposes: - caching read() data from files - caching mmap()ed data and executable files - swap cache When your system is both deep in swap and high on cache, it probably means that a lot of the swaped data is being cached, making for more efficient swapping than possible with the 2.0 kernel. ============================================================== pagetable_cache: The kernel keeps a number of page tables in a per-processor cache (this helps a lot on SMP systems). The cache size for each processor will be between the low and the high value. On a low-memory, single CPU system you can safely set these values to 0 so you don't waste the memory. On SMP systems it is used so that the system can do fast pagetable allocations without having to aquire the kernel memory lock. For large systems, the settings are probably OK. For normal systems they won't hurt a bit. For small systems (<16MB ram) it might be advantageous to set both values to 0.