mirror of
https://github.com/torvalds/linux.git
synced 2024-11-22 12:11:40 +00:00
A mirror of the official Linux kernel repository just in case
fd7732e033
When we find an existing lock which conflicts with a request, and the request wants to wait, we currently add the request to a list. When the lock is removed, the whole list is woken. This can cause the thundering-herd problem. To reduce the problem, we make use of the (new) fact that a pending request can itself have a list of blocked requests. When we find a conflict, we look through the existing blocked requests. If any one of them blocks the new request, the new request is attached below that request, otherwise it is added to the list of blocked requests, which are now known to be mutually non-conflicting. This way, when the lock is released, only a set of non-conflicting locks will be woken, the rest can stay asleep. If the lock request cannot be granted and the request needs to be requeued, all the other requests it blocks will then be woken To make this more concrete: If you have a many-core machine, and have many threads all wanting to briefly lock a give file (udev is known to do this), you can get quite poor performance. When one thread releases a lock, it wakes up all other threads that are waiting (classic thundering-herd) - one will get the lock and the others go to sleep. When you have few cores, this is not very noticeable: by the time the 4th or 5th thread gets enough CPU time to try to claim the lock, the earlier threads have claimed it, done what was needed, and released. So with few cores, many of the threads don't end up contending. With 50+ cores, lost of threads can get the CPU at the same time, and the contention can easily be measured. This patchset creates a tree of pending lock requests in which siblings don't conflict and each lock request does conflict with its parent. When a lock is released, only requests which don't conflict with each other a woken. Testing shows that lock-acquisitions-per-second is now fairly stable even as the number of contending process goes to 1000. Without this patch, locks-per-second drops off steeply after a few 10s of processes. There is a small cost to this extra complexity. At 20 processes running a particular test on 72 cores, the lock acquisitions per second drops from 1.8 million to 1.4 million with this patch. For 100 processes, this patch still provides 1.4 million while without this patch there are about 700,000. Reported-and-tested-by: Martin Wilck <mwilck@suse.de> Signed-off-by: NeilBrown <neilb@suse.com> Reviewed-by: J. Bruce Fields <bfields@redhat.com> Signed-off-by: Jeff Layton <jlayton@kernel.org> |
||
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| firmware | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.