Regarding vfs.lorunningspace and vfs.hirunningspace... On 2010-Jul-15 13:52:43 -0500, Alan Cox <alan.l....@gmail.com> wrote: >Keep in mind that we still run on some fairly small systems with limited I/O >capabilities, e.g., a typical arm platform. More generally, with the range >of systems that FreeBSD runs on today, any particular choice of constants is >going to perform poorly for someone. If nothing else, making these sysctls >a function of the buffer cache size is probably better than any particular >constants.
That sounds reasonable but brings up a related issue - the buffer cache. Given the unified VM system no longer needs a traditional Unix buffer cache, what is the buffer cache still used for? Is the current tuning formula still reasonable (for virtually all current systems it's basically 10MB + 10% RAM)? How can I measure the effectiveness of the buffer cache? The buffer cache size is also very tightly constrained (vfs.hibufspace and vfs.lobufspace differ by 64KB) and at least one of the underlying tuning parameters have comments at variance with current reality: In <sys/param.h>: * MAXBSIZE - Filesystems are made out of blocks of at most MAXBSIZE bytes * per block. MAXBSIZE may be made larger without effecting ... * * BKVASIZE - Nominal buffer space per buffer, in bytes. BKVASIZE is the ... * The default is 16384, roughly 2x the block size used by a * normal UFS filesystem. */ #define MAXBSIZE 65536 /* must be power of 2 */ #define BKVASIZE 16384 /* must be power of 2 */ There's no mention of the 64KiB limit in newfs(8) and I recall seeing occasional comments from people who have either tried or suggested trying larger blocksizes. Likewise, the default UFS blocksize has been 16KiB for quite a while. Are the comments still valid and, if so, should BKVASIZE be doubled to 32768 and a suitable note added to newfs(8) regarding the maximum block size? -- Peter Jeremy
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