minix/servers/fs/cache2.c
2005-08-22 15:23:47 +00:00

126 lines
4.2 KiB
C

/* Second level block cache to supplement the file system cache. The block
* cache of a 16-bit Minix system is very small, too small to prevent trashing.
* A generic 32-bit system also doesn't have a very large cache to allow it
* to run on systems with little memory. On a system with lots of memory one
* can use the RAM disk as a read-only second level cache. Any blocks pushed
* out of the primary cache are cached on the RAM disk. This code manages the
* second level cache. The cache is a simple FIFO where old blocks are put
* into and drop out at the other end. Must be searched backwards.
*
* The entry points into this file are:
* init_cache2: initialize the second level cache
* get_block2: get a block from the 2nd level cache
* put_block2: store a block in the 2nd level cache
* invalidate2: remove all the cache blocks on some device
*/
#include "fs.h"
#include <minix/com.h>
#include "buf.h"
#if ENABLE_CACHE2
#define MAX_BUF2 (256 * sizeof(char *))
PRIVATE struct buf2 { /* 2nd level cache per block administration */
block_t b2_blocknr; /* block number */
dev_t b2_dev; /* device number */
u16_t b2_count; /* count of in-cache block groups */
} buf2[MAX_BUF2];
PRIVATE unsigned nr_buf2; /* actual cache size */
PRIVATE unsigned buf2_idx; /* round-robin reuse index */
#define hash2(block) ((unsigned) ((block) & (MAX_BUF2 - 1)))
/*===========================================================================*
* init_cache2 *
*===========================================================================*/
PUBLIC void init_cache2(size)
unsigned long size;
{
/* Initialize the second level disk buffer cache of 'size' blocks. */
nr_buf2 = size > MAX_BUF2 ? MAX_BUF2 : (unsigned) size;
}
/*===========================================================================*
* get_block2 *
*===========================================================================*/
PUBLIC int get_block2(bp, only_search)
struct buf *bp; /* buffer to get from the 2nd level cache */
int only_search; /* if NO_READ, do nothing, else act normal */
{
/* Fill a buffer from the 2nd level cache. Return true iff block acquired. */
unsigned b;
struct buf2 *bp2;
/* If the block wanted is in the RAM disk then our game is over. */
if (bp->b_dev == DEV_RAM) nr_buf2 = 0;
/* Cache enabled? NO_READ? Any blocks with the same hash key? */
if (nr_buf2 == 0 || only_search == NO_READ
|| buf2[hash2(bp->b_blocknr)].b2_count == 0) return(0);
/* Search backwards (there may be older versions). */
b = buf2_idx;
for (;;) {
if (b == 0) b = nr_buf2;
bp2 = &buf2[--b];
if (bp2->b2_blocknr == bp->b_blocknr && bp2->b2_dev == bp->b_dev) break;
if (b == buf2_idx) return(0);
}
/* Block is in the cache, get it. */
if (dev_io(DEV_READ, DEV_RAM, FS_PROC_NR, bp->b_data,
(off_t) b * BLOCK_SIZE, BLOCK_SIZE, 0) == BLOCK_SIZE) {
return(1);
}
return(0);
}
/*===========================================================================*
* put_block2 *
*===========================================================================*/
PUBLIC void put_block2(bp)
struct buf *bp; /* buffer to store in the 2nd level cache */
{
/* Store a buffer into the 2nd level cache. */
unsigned b;
struct buf2 *bp2;
if (nr_buf2 == 0) return; /* no 2nd level cache */
b = buf2_idx++;
if (buf2_idx == nr_buf2) buf2_idx = 0;
bp2 = &buf2[b];
if (dev_io(DEV_WRITE, DEV_RAM, FS_PROC_NR, bp->b_data,
(off_t) b * BLOCK_SIZE, BLOCK_SIZE, 0) == BLOCK_SIZE) {
if (bp2->b2_dev != NO_DEV) buf2[hash2(bp2->b2_blocknr)].b2_count--;
bp2->b2_dev = bp->b_dev;
bp2->b2_blocknr = bp->b_blocknr;
buf2[hash2(bp2->b2_blocknr)].b2_count++;
}
}
/*===========================================================================*
* invalidate2 *
*===========================================================================*/
PUBLIC void invalidate2(device)
dev_t device;
{
/* Invalidate all blocks from a given device in the 2nd level cache. */
unsigned b;
struct buf2 *bp2;
for (b = 0; b < nr_buf2; b++) {
bp2 = &buf2[b];
if (bp2->b2_dev == device) {
bp2->b2_dev = NO_DEV;
buf2[hash2(bp2->b2_blocknr)].b2_count--;
}
}
}
#endif /* ENABLE_CACHE2 */