This new interface allows for slot reservation to avoid additional
copy-overhead from consumer. The primary use-case is for the type-specialized
variant of ck_ring. The initial patch-set does not migrate enqueue and
dequeue to be implemented in terms of reserve and commit but will be a future
commit.
There was a silly typo and circular dependency introduced in the migration.
Thanks to Sarah Edkins <sedkins@backtrace.io> for letting me borrow her laptop
to investigate.
These tests check for sane behavior in the presence of new
maps being created for the hash set. They require the presence
of SMR.
For the life time of the growth_spmc tests, disable deallocation.
ck_ec implements 32- and (on 64 bit platforms) 64- bit event
counts. Event counts let us easily integrate OS-level blocking (e.g.,
futexes) in lock-free protocols. Waking up waiters only locks in the
OS kernel, and does not happen at all when no waiter is blocked.
Waiters only block conditionally, if the event count's value is
still equal to some prior value.
ck_ec supports multiple producers (wakers) and consumers (waiters),
and, on x86-TSO, has a more efficient specialisation for single
producer mode. In the latter mode, the overhead compared to a version
counter is on the order of 2-3 cycles and 1-2 instructions, in the
fast path. The slow path, when there are threads blocked on the event
count, consists of one additional atomic instruction and a futex
syscall.
Similarly, the fast path for consumers, when an update comes quickly,
has no overhead compared to spinning on a read-only counter. After
a few thousand cycles, consumers (waiters) enter the slow path with
one atomic instruction and a few blocking syscalls.
The single-producer specialisation requires the x86-TSO memory model,
x86's non-atomic read-modify-write instructions, and, ideally a
futex-like OS abstraction. On !x86/x86_64 platforms, single producer
increments fall back to the multiple producer code path.
Fixes https://github.com/concurrencykit/ck/issues/79
On FreeBSD, atomic operations in the kernel must access the nucleus
address space. Userland may use either the atomic instruction set
which goes without an ASI (address space identifier) or specify the
primary address space.
To avoid hardcoding the address space here, we grab the corresponding
identifier from the appropriate machine header but also only for the
kernel so the namespace doesn't get polluted for userland.
This work is from jtl@FreeBSD.org. FreeBSD expects to call ck_epoch_poll
from a record that is in an active section. Previously, it was
considered an API violation to call write-side functions while in a read
section.
This is now permitted for poll as we we serialize behind the global
epoch counter.
Note that these functions are not reentrant. In the case of the
FreeBSD kernel, all these functions are called with preemption disabled.
This work is from Jonathan T. Looney from the FreeBSD project
(jtl@).
The return value of ck_epoch_poll has also changed. It returns false
only if the epoch counter has not progressed, if no memory was reclaimed
(in other words, no forward progress) and / or not all threads have been
observed in a quiescent state (grace period).
Below are his notes:
Epoch calls are stored in a 4-bucket hash table. The 4-bucket hash table
allows for calls to be stored for three epochs: the current epoch and
two previous ones. The comments at the beginning of ck_epoch.c explain
why this is necessary.
When there are active threads, ck_epoch_poll_deferred() current runs the
epoch calls for the current global epoch + 1. Because of modulo
arithmetic, this is equivalent to running the calls for epoch - 3.
However, this means that ck_epoch_poll_deferred() is waiting
unnecessarily long to run epoch calls.
Further, there could be races in incrementing the global epoch. Imagine
all active threads have observed epoch n. CPU 0 sees this. It increments
the global epoch to (n + 1). It runs the epoch calls for (n - 3). Now,
CPU 1 checks. It sees that there are active threads which have not yet
observed the new global epoch (n + 1). In this case,
ck_epoch_poll_deferred() will return without running any epochs. In the
worst case (CPU 1 continually losing the race), these epoch calls could
be deferred indefinitely.
To fix this, always run any epoch calls for global epoch - 2. Further,
if all active threads have observed the global epoch, run epoch calls
for global epoch - 1.
The global epoch is only incremented when all active threads have
observed it. Therefore, all active threads must always have observed
global epoch - 1 or the current global epoch. Accordingly, it is safe to
always run epoch calls for global epoch - 2.
Further, if all active threads have observed the global epoch, it is
safe to run epoch calls for global epoch - 1.
Paul Khuong
Samy Al Bahra
Samy Al Bahra
Olivier Houchard
Olivier Houchard
Fedor Korotkov
Li-Wen Hsu
Thibault Martinez
Skyb0rg007
Paul Khuong
Marius Strobl