Hi Coly,
>>
>> Hello Coly,
>>
>>> On 2018/4/12 2:38 PM, tang.junhui@xxxxxxxxxx wrote:
>>>> From: Tang Junhui <tang.junhui@xxxxxxxxxx>
>>>>
>>>> In GC thread, we record the latest GC key in gc_done, which is expected
>>>> to be used for incremental GC, but in currently code, we didn't realize
>>>> it. When GC runs, front side IO would be blocked until the GC over, it
>>>> would be a long time if there is a lot of btree nodes.
>>>>
>>>> This patch realizes incremental GC, the main ideal is that, when there
>>>> are front side I/Os, after GC some nodes (100), we stop GC, release locker
>>>> of the btree node, and go to process the front side I/Os for some times
>>>> (100 ms), then go back to GC again.
>>>>
>>>> By this patch, when we doing GC, I/Os are not blocked all the time, and
>>>> there is no obvious I/Os zero jump problem any more.
>>>>
>>>> Signed-off-by: Tang Junhui <tang.junhui@xxxxxxxxxx>
>>>
>>> Hi Junhui,
>>>
>>> I reply my comments in line,
>> Thanks for your comments in advance.
>>>
>>>> ---
>>>> drivers/md/bcache/bcache.h | 5 +++++
>>>> drivers/md/bcache/btree.c | 15 ++++++++++++++-
>>>> drivers/md/bcache/request.c | 3 +++
>>>> 3 files changed, 22 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
>>>> index 843877e..ab4c9ca 100644
>>>> --- a/drivers/md/bcache/bcache.h
>>>> +++ b/drivers/md/bcache/bcache.h
>>>> @@ -445,6 +445,7 @@ struct cache {
>>>>
>>>> struct gc_stat {
>>>> size_t nodes;
>>>> + size_t nodes_pre;
>>>> size_t key_bytes;
>>>>
>>>> size_t nkeys;
>>>> @@ -568,6 +569,10 @@ struct cache_set {
>>>> */
>>>> atomic_t rescale;
>>>> /*
>>>> + * used for GC, identify if any front side I/Os is inflight
>>>> + */
>>>> + atomic_t io_inflight;
>>>
>>> Could you please to rename the above variable to something like
>>> search_inflight ? Just to be more explicit, considering we have many
>>> types of io requests.
>> OK, It looks better.
>>>
>>>> + /*
>>>> * When we invalidate buckets, we use both the priority and the amount
>>>> * of good data to determine which buckets to reuse first - to weight
>>>> * those together consistently we keep track of the smallest nonzero
>>>> diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
>>>> index 81e8dc3..b36d276 100644
>>>> --- a/drivers/md/bcache/btree.c
>>>> +++ b/drivers/md/bcache/btree.c
>>>> @@ -90,6 +90,9 @@
>>>>
>>>> #define MAX_NEED_GC 64
>>>> #define MAX_SAVE_PRIO 72
>>>> +#define MIN_GC_NODES 100
>>>> +#define GC_SLEEP_TIME 100
>>>
>>> How about naming the above macro as GC_SLEEP_MS ? So people may
>>> understand the sleep time is 100ms without checking more context.
>> OK, It looks better.
>>>> +
>>>>
>>>> #define PTR_DIRTY_BIT (((uint64_t) 1 << 36))
>>>>
>>>> @@ -1581,6 +1584,13 @@ static int btree_gc_recurse(struct btree *b, struct btree_op *op,
>>>> memmove(r + 1, r, sizeof(r[0]) * (GC_MERGE_NODES - 1));
>>>> r->b = NULL;
>>>>
>>>> + if (atomic_read(&b->c->io_inflight) &&
>>>> + gc->nodes >= gc->nodes_pre + MIN_GC_NODES) {
>>>
>>> On 32bit machines, gc->nodes is a 32bit count, if it is overflowed the
>>> above check will be false for a very long time, and in some condition it
>>> might be always false after gc->nodes overflowed.
>>>
>>> How about adding the following check before the if() statement ?
>>
>>> /* in case gc->nodes is overflowed */
>>> if (gc->nodes_pre < gc->nodes)
>>> gc->noeds_pre = gc->nodes;
>>>
>> I think 32bit is big enough, which can express a value of billions,
>> but the number of nodes is usually around tens of thousands.
>> Also gc->nodes and gc->nodes_pre were set to zero when GC begin each time.
>>
>> How do you think about?
>
>Oh, I see. Then I don't worry here. The patch is OK to me.
Thanks Coly, Would you mind if I add you as a reveiw-by in the v2 patch?
Thanks.
Tang Junhui
