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Clarify some comments.

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- the intention for my_getevents syscall is now better explained,
why are we using it (to be able to interrupt io_getevents syscall via
io_destroy()).

- Fix comment for MAX_EVENTS in getevent_thread_routine.
MAX_EVENTS is more of less arbitrary constant, chosen such that events array
is big enough to get multiple simultaneous io completions, but small
enough so it does not blow the thread's stack.
This commit is contained in:
Vladislav Vaintroub 2020-11-27 01:09:11 +01:00
parent 5bb5d4ad3a
commit 1435f35bda
1 changed files with 25 additions and 5 deletions
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30
tpool/aio_linux.cc
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@ -23,7 +23,7 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02111 - 1301 USA*/
# include <sys/syscall.h>
/**
Invoke the io_getevents() system call.
Invoke the io_getevents() system call, without timeout parameter.
@param ctx context from io_setup()
@param min_nr minimum number of completion events to wait for
@ -37,7 +37,25 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02111 - 1301 USA*/
The libaio code for dereferencing ctx would occasionally trigger
SIGSEGV if io_destroy() was concurrently invoked from another thread.
Hence, we use the raw system call.
Hence, we have to use the raw system call.
WHY are we doing this at all?
Because we want io_destroy() from another thread to interrupt io_getevents().
And, WHY do we want io_destroy() from another thread to interrupt
io_getevents()?
Because there is no documented, libaio-friendly and race-condition-free way to
interrupt io_getevents(). io_destroy() coupled with raw syscall seemed to work
for us so far.
Historical note : in the past, we used io_getevents with timeouts. We'd wake
up periodically, check for shutdown flag, return from the main routine.
This was admittedly safer, yet it did cost periodic wakeups, which we are not
willing to do anymore.
@note we also rely on the undocumented property, that io_destroy(ctx)
will make this version of io_getevents return EINVAL.
*/
static int my_getevents(io_context_t ctx, long min_nr, long nr, io_event *ev)
{
@ -77,10 +95,12 @@ class aio_linux final : public aio
static void getevent_thread_routine(aio_linux *aio)
{
/* We collect this many events at a time. os_aio_init() would
multiply OS_AIO_N_PENDING_THREADS by the number of read and write threads
and ultimately pass it to io_setup() via thread_pool::configure_aio(). */
/*
We collect events in small batches to hopefully reduce the
number of system calls.
*/
constexpr unsigned MAX_EVENTS= 256;
io_event events[MAX_EVENTS];
for (;;)
{