litmus-rt.git - The LITMUS^RT kernel.

The LITMUS^RT kernel.

Bjoern Brandenburg

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author	Linus Torvalds <torvalds@linux-foundation.org>	2010-11-01 07:53:19 -0400
committer	Linus Torvalds <torvalds@linux-foundation.org>	2010-11-01 07:53:19 -0400
commit	b3b2b6351a3f3342e6669b5c2defc85f6af75ee3 (patch)
tree	9b47ae7b0111ab024d719214d2745b128db48beb /include/linux/flat.h
parent	90ae83f7fc32733e5829d806306c0c590f1a383f (diff)
parent	395bc5121828e570bd4bb6455c82b80e2f2b05b4 (diff)

Merge branch 'for-linus' of git://www.jni.nu/cris

* 'for-linus' of git://www.jni.nu/cris: CRIS: Add paths for CRISv10 serial driver CRIS: Fix RS485 delay handling. Add missing "struct" to in sizeof.

Diffstat (limited to 'include/linux/flat.h')

0 files changed, 0 insertions, 0 deletions

quest() to end_that_request_last()). Field 9 -- # of I/Os currently in progress The only field that should go to zero. Incremented as requests are given to appropriate struct request_queue and decremented as they finish. Field 10 -- # of milliseconds spent doing I/Os This field increases so long as field 9 is nonzero. Field 11 -- weighted # of milliseconds spent doing I/Os This field is incremented at each I/O start, I/O completion, I/O merge, or read of these stats by the number of I/Os in progress (field 9) times the number of milliseconds spent doing I/O since the last update of this field. This can provide an easy measure of both I/O completion time and the backlog that may be accumulating. To avoid introducing performance bottlenecks, no locks are held while modifying these counters. This implies that minor inaccuracies may be introduced when changes collide, so (for instance) adding up all the read I/Os issued per partition should equal those made to the disks ... but due to the lack of locking it may only be very close. In 2.6, there are counters for each CPU, which make the lack of locking almost a non-issue. When the statistics are read, the per-CPU counters are summed (possibly overflowing the unsigned long variable they are summed to) and the result given to the user. There is no convenient user interface for accessing the per-CPU counters themselves. Disks vs Partitions ------------------- There were significant changes between 2.4 and 2.6 in the I/O subsystem. As a result, some statistic information disappeared. The translation from a disk address relative to a partition to the disk address relative to the host disk happens much earlier. All merges and timings now happen at the disk level rather than at both the disk and partition level as in 2.4. Consequently, you'll see a different statistics output on 2.6 for partitions from that for disks. There are only *four* fields available for partitions on 2.6 machines. This is reflected in the examples above. Field 1 -- # of reads issued This is the total number of reads issued to this partition. Field 2 -- # of sectors read This is the total number of sectors requested to be read from this partition. Field 3 -- # of writes issued This is the total number of writes issued to this partition. Field 4 -- # of sectors written This is the total number of sectors requested to be written to this partition. Note that since the address is translated to a disk-relative one, and no record of the partition-relative address is kept, the subsequent success or failure of the read cannot be attributed to the partition. In other words, the number of reads for partitions is counted slightly before time of queuing for partitions, and at completion for whole disks. This is a subtle distinction that is probably uninteresting for most cases. More significant is the error induced by counting the numbers of reads/writes before merges for partitions and after for disks. Since a typical workload usually contains a lot of successive and adjacent requests, the number of reads/writes issued can be several times higher than the number of reads/writes completed. In 2.6.25, the full statistic set is again available for partitions and disk and partition statistics are consistent again. Since we still don't keep record of the partition-relative address, an operation is attributed to the partition which contains the first sector of the request after the eventual merges. As requests can be merged across partition, this could lead to some (probably insignificant) inaccuracy. Additional notes ---------------- In 2.6, sysfs is not mounted by default. If your distribution of Linux hasn't added it already, here's the line you'll want to add to your /etc/fstab: none /sys sysfs defaults 0 0 In 2.6, all disk statistics were removed from /proc/stat. In 2.4, they appear in both /proc/partitions and /proc/stat, although the ones in /proc/stat take a very different format from those in /proc/partitions (see proc(5), if your system has it.) -- ricklind@us.ibm.com


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