Date: Sat, 25 Jul 1998 00:43:28 +0000 (GMT) From: Terry Lambert <tlambert@primenet.com> To: graaf@surf.IAE.nl (Edwin de Graaf) Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: How do I measure swap activity? Message-ID: <199807250043.RAA18387@usr06.primenet.com> In-Reply-To: <199807241249.OAA04892@surf.IAE.nl> from "Edwin de Graaf" at Jul 24, 98 02:49:40 pm
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> I am looking for a good way to measure how much disk activity is caused by > pages being swapped in and out from disk. In the systat -vmstat display > there are two kinds of pagers, the VN pager and the SWAP pager. Do both of > these cause a disk access? What is the difference between the two? Unmodified data pages and unmodified text pages are swapped into memory from the program image in a filesystem, using the VN pager to access the filesystem. In addition, pages backed by mmap'ed files are swapped in and out via the vnode pager. Dirty data pages are backed by swap, and are swapped out using the SWAP pager. > On the other hand I have vmstat which shows the number of page faults, is > this the sum of the VN and SWAP pagers? Does each page fault cause a fixed > block (4096 bytes?) to be read from disk? Pages are aggressively cached in FreeBSD. A fault occurs when a mapped page is referenced, but not present. The difference between thisn and the infamous Windows 95 "page not present" error is that the error is handled by acquiring an anonymous page, and filling it from the swap store backing the object. In general, this is either a vnode or swap region. > Pointers to documents describing these would also be most welcome. You should look at the -current list archives at FreeBSD.org where a document describing the VM architecture in good detail was recently reposted. Terry Lambert terry@lambert.org --- Any opinions in this posting are my own and not those of my present or previous employers. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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