Paolo Bonzini <pbonz...@redhat.com> writes:
> On 13/05/2015 17:18, Alexander Yarygin wrote:
>> After the commit 9b536adc ("block: acquire AioContext in
>> bdrv_drain_all()") the aio_poll() function got called for every
>> BlockDriverState, in assumption that every device may have its own
>> AioContext. The bdrv_drain_all() function is called in each
>> virtio_reset() call,
>
> ... which should actually call bdrv_drain(). Can you fix that?
>
I thought about it, but couldn't come to conclusion that it's safe. The
comment above bdrv_drain_all() states "... it is not possible to have a
function to drain a single device's I/O queue.", besides that what if we
have several virtual disks that share host file?
Or I'm wrong and it's ok to do?
>> which in turn is called for every virtio-blk
>> device on initialization, so we got aio_poll() called
>> 'length(device_list)^2' times.
>>
>> If we have thousands of disks attached, there are a lot of
>> BlockDriverStates but only a few AioContexts, leading to tons of
>> unnecessary aio_poll() calls. For example, startup times with 1000 disks
>> takes over 13 minutes.
>>
>> This patch changes the bdrv_drain_all() function allowing it find shared
>> AioContexts and to call aio_poll() only for unique ones. This results in
>> much better startup times, e.g. 1000 disks do come up within 5 seconds.
>
> I'm not sure this patch is correct. You may have to call aio_poll
> multiple times before a BlockDriverState is drained.
>
> Paolo
>
Ah, right. We need second loop, something like this:
@@ -2030,20 +2033,33 @@ void bdrv_drain(BlockDriverState *bs)
void bdrv_drain_all(void)
{
/* Always run first iteration so any pending completion BHs run */
- bool busy = true;
+ bool busy = true, pending = false;
BlockDriverState *bs;
+ GList *aio_ctxs = NULL, *ctx;
+ AioContext *aio_context;
while (busy) {
busy = false;
QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
- AioContext *aio_context = bdrv_get_aio_context(bs);
+ aio_context = bdrv_get_aio_context(bs);
aio_context_acquire(aio_context);
busy |= bdrv_drain_one(bs);
aio_context_release(aio_context);
+ if (!aio_ctxs || !g_list_find(aio_ctxs, aio_context))
+ aio_ctxs = g_list_append(aio_ctxs, aio_context);
+ }
+ pending = busy;
+
+ for (ctx = aio_ctxs; ctx != NULL; ctx = ctx->next) {
+ aio_context = ctx->data;
+ aio_context_acquire(aio_context);
+ busy |= aio_poll(aio_context, pending);
+ aio_context_release(aio_context);
}
}
+ g_list_free(aio_ctxs);
}
That looks quite ugly for me and breaks consistence of bdrv_drain_one()
since it doesn't call aio_poll() anymore...
>> Cc: Christian Borntraeger <borntrae...@de.ibm.com>
>> Cc: Cornelia Huck <cornelia.h...@de.ibm.com>
>> Cc: Kevin Wolf <kw...@redhat.com>
>> Cc: Paolo Bonzini <pbonz...@redhat.com>
>> Cc: Stefan Hajnoczi <stefa...@redhat.com>
>> Signed-off-by: Alexander Yarygin <yary...@linux.vnet.ibm.com>
>> ---
>> block.c | 13 +++++++++++--
>> 1 file changed, 11 insertions(+), 2 deletions(-)
>>
>> diff --git a/block.c b/block.c
>> index f2f8ae7..7414815 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -1994,7 +1994,6 @@ static bool bdrv_drain_one(BlockDriverState *bs)
>> bdrv_flush_io_queue(bs);
>> bdrv_start_throttled_reqs(bs);
>> bs_busy = bdrv_requests_pending(bs);
>> - bs_busy |= aio_poll(bdrv_get_aio_context(bs), bs_busy);
>> return bs_busy;
>> }
>>
>> @@ -2010,8 +2009,12 @@ static bool bdrv_drain_one(BlockDriverState *bs)
>> */
>> void bdrv_drain(BlockDriverState *bs)
>> {
>> - while (bdrv_drain_one(bs)) {
>> + bool busy = true;
>> +
>> + while (busy) {
>> /* Keep iterating */
>> + busy = bdrv_drain_one(bs);
>> + busy |= aio_poll(bdrv_get_aio_context(bs), busy);
>> }
>> }
>>
>> @@ -2032,6 +2035,7 @@ void bdrv_drain_all(void)
>> /* Always run first iteration so any pending completion BHs run */
>> bool busy = true;
>> BlockDriverState *bs;
>> + GList *aio_ctxs = NULL;
>>
>> while (busy) {
>> busy = false;
>> @@ -2041,9 +2045,14 @@ void bdrv_drain_all(void)
>>
>> aio_context_acquire(aio_context);
>> busy |= bdrv_drain_one(bs);
>> + if (!aio_ctxs || !g_list_find(aio_ctxs, aio_context)) {
>> + busy |= aio_poll(aio_context, busy);
>> + aio_ctxs = g_list_append(aio_ctxs, aio_context);
>> + }
>> aio_context_release(aio_context);
>> }
>> }
>> + g_list_free(aio_ctxs);
>> }
>>
>> /* make a BlockDriverState anonymous by removing from bdrv_state and
>>
