13.08.2019 17:23, Max Reitz wrote:
> On 13.08.19 16:14, Vladimir Sementsov-Ogievskiy wrote:
>> 12.08.2019 19:37, Vladimir Sementsov-Ogievskiy wrote:
>>> 12.08.2019 19:11, Max Reitz wrote:
>>>> On 12.08.19 17:47, Vladimir Sementsov-Ogievskiy wrote:
>>>>> 12.08.2019 18:10, Max Reitz wrote:
>>>>>> On 10.08.19 21:31, Vladimir Sementsov-Ogievskiy wrote:
>>>>>>> backup_cow_with_offload can transfer more than one cluster. Let
>>>>>>> backup_cow_with_bounce_buffer behave similarly. It reduces the number
>>>>>>> of IO requests, since there is no need to copy cluster by cluster.
>>>>>>>
>>>>>>> Logic around bounce_buffer allocation changed: we can't just allocate
>>>>>>> one-cluster-sized buffer to share for all iterations. We can't also
>>>>>>> allocate buffer of full-request length it may be too large, so
>>>>>>> BACKUP_MAX_BOUNCE_BUFFER is introduced. And finally, allocation logic
>>>>>>> is to allocate a buffer sufficient to handle all remaining iterations
>>>>>>> at the point where we need the buffer for the first time.
>>>>>>>
>>>>>>> Bonus: get rid of pointer-to-pointer.
>>>>>>>
>>>>>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
>>>>>>> ---
>>>>>>> block/backup.c | 65
>>>>>>> +++++++++++++++++++++++++++++++-------------------
>>>>>>> 1 file changed, 41 insertions(+), 24 deletions(-)
>>>>>>>
>>>>>>> diff --git a/block/backup.c b/block/backup.c
>>>>>>> index d482d93458..65f7212c85 100644
>>>>>>> --- a/block/backup.c
>>>>>>> +++ b/block/backup.c
>>>>>>> @@ -27,6 +27,7 @@
>>>>>>> #include "qemu/error-report.h"
>>>>>>> #define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16)
>>>>>>> +#define BACKUP_MAX_BOUNCE_BUFFER (64 * 1024 * 1024)
>>>>>>> typedef struct CowRequest {
>>>>>>> int64_t start_byte;
>>>>>>> @@ -98,44 +99,55 @@ static void cow_request_end(CowRequest *req)
>>>>>>> qemu_co_queue_restart_all(&req->wait_queue);
>>>>>>> }
>>>>>>> -/* Copy range to target with a bounce buffer and return the bytes
>>>>>>> copied. If
>>>>>>> - * error occurred, return a negative error number */
>>>>>>> +/*
>>>>>>> + * Copy range to target with a bounce buffer and return the bytes
>>>>>>> copied. If
>>>>>>> + * error occurred, return a negative error number
>>>>>>> + *
>>>>>>> + * @bounce_buffer is assumed to enough to store
>>>>>>
>>>>>> s/to/to be/
>>>>>>
>>>>>>> + * MIN(BACKUP_MAX_BOUNCE_BUFFER, @end - @start) bytes
>>>>>>> + */
>>>>>>> static int coroutine_fn
>>>>>>> backup_cow_with_bounce_buffer(BackupBlockJob *job,
>>>>>>> int64_t start,
>>>>>>> int64_t end,
>>>>>>> bool
>>>>>>> is_write_notifier,
>>>>>>> bool
>>>>>>> *error_is_read,
>>>>>>> - void
>>>>>>> **bounce_buffer)
>>>>>>> + void
>>>>>>> *bounce_buffer)
>>>>>>> {
>>>>>>> int ret;
>>>>>>> BlockBackend *blk = job->common.blk;
>>>>>>> - int nbytes;
>>>>>>> + int nbytes, remaining_bytes;
>>>>>>> int read_flags = is_write_notifier ? BDRV_REQ_NO_SERIALISING :
>>>>>>> 0;
>>>>>>> assert(QEMU_IS_ALIGNED(start, job->cluster_size));
>>>>>>> - bdrv_reset_dirty_bitmap(job->copy_bitmap, start,
>>>>>>> job->cluster_size);
>>>>>>> - nbytes = MIN(job->cluster_size, job->len - start);
>>>>>>> - if (!*bounce_buffer) {
>>>>>>> - *bounce_buffer = blk_blockalign(blk, job->cluster_size);
>>>>>>> - }
>>>>>>> + bdrv_reset_dirty_bitmap(job->copy_bitmap, start, end - start);
>>>>>>> + nbytes = MIN(end - start, job->len - start);
>>>>>>> - ret = blk_co_pread(blk, start, nbytes, *bounce_buffer, read_flags);
>>>>>>> - if (ret < 0) {
>>>>>>> - trace_backup_do_cow_read_fail(job, start, ret);
>>>>>>> - if (error_is_read) {
>>>>>>> - *error_is_read = true;
>>>>>>> +
>>>>>>> + remaining_bytes = nbytes;
>>>>>>> + while (remaining_bytes) {
>>>>>>> + int chunk = MIN(BACKUP_MAX_BOUNCE_BUFFER, remaining_bytes);
>>>>>>> +
>>>>>>> + ret = blk_co_pread(blk, start, chunk, bounce_buffer,
>>>>>>> read_flags);
>>>>>>> + if (ret < 0) {
>>>>>>> + trace_backup_do_cow_read_fail(job, start, ret);
>>>>>>> + if (error_is_read) {
>>>>>>> + *error_is_read = true;
>>>>>>> + }
>>>>>>> + goto fail;
>>>>>>> }
>>>>>>> - goto fail;
>>>>>>> - }
>>>>>>> - ret = blk_co_pwrite(job->target, start, nbytes, *bounce_buffer,
>>>>>>> - job->write_flags);
>>>>>>> - if (ret < 0) {
>>>>>>> - trace_backup_do_cow_write_fail(job, start, ret);
>>>>>>> - if (error_is_read) {
>>>>>>> - *error_is_read = false;
>>>>>>> + ret = blk_co_pwrite(job->target, start, chunk, bounce_buffer,
>>>>>>> + job->write_flags);
>>>>>>> + if (ret < 0) {
>>>>>>> + trace_backup_do_cow_write_fail(job, start, ret);
>>>>>>> + if (error_is_read) {
>>>>>>> + *error_is_read = false;
>>>>>>> + }
>>>>>>> + goto fail;
>>>>>>> }
>>>>>>> - goto fail;
>>>>>>> +
>>>>>>> + start += chunk;
>>>>>>> + remaining_bytes -= chunk;
>>>>>>> }
>>>>>>> return nbytes;
>>>>>>> @@ -301,9 +313,14 @@ static int coroutine_fn
>>>>>>> backup_do_cow(BackupBlockJob *job,
>>>>>>> }
>>>>>>> }
>>>>>>> if (!job->use_copy_range) {
>>>>>>> + if (!bounce_buffer) {
>>>>>>> + size_t len = MIN(BACKUP_MAX_BOUNCE_BUFFER,
>>>>>>> + MAX(dirty_end - start, end -
>>>>>>> dirty_end));
>>>>>>> + bounce_buffer = blk_try_blockalign(job->common.blk,
>>>>>>> len);
>>>>>>> + }
>>>>>>
>>>>>> If you use _try_, you should probably also check whether it succeeded.
>>>>>
>>>>> Oops, you are right, of course.
>>>>>
>>>>>>
>>>>>> Anyway, I wonder whether it’d be better to just allocate this buffer
>>>>>> once per job (the first time we get here, probably) to be of size
>>>>>> BACKUP_MAX_BOUNCE_BUFFER and put it into BackupBlockJob. (And maybe add
>>>>>> a buf-size parameter similar to what the mirror jobs have.)
>>>>>>
>>>>>
>>>>> Once per job will not work, as we may have several guest writes in
>>>>> parallel and therefore
>>>>> several parallel copy-before-write operations.
>>>>
>>>> Hm. I’m not quite happy with that because if the guest just issues many
>>>> large discards in parallel, this means that qemu will allocate a large
>>>> amount of memory.
>>>>
>>>> It would be nice if there was a simple way to keep track of the total
>>>> memory usage and let requests yield if they would exceed it.
>>>
>>> Agree, it should be fixed anyway.
>>>
>>
>>
>> But still..
>>
>> Synchronous mirror allocates full-request buffers on guest write. Is it
>> correct?
>>
>> If we assume that it is correct to double memory usage of guest operations,
>> than for backup
>> the problem is only in write_zero and discard where guest-assumed memory
>> usage should be zero.
>
> Well, but that is the problem. I didn’t say anything in v2, because I
> only thought of normal writes and I found it fine to double the memory
> usage there (a guest won’t issue huge write requests in parallel). But
> discard/write-zeroes are a different matter.
>
>> And if we should distinguish writes from write_zeroes and discard, it's
>> better to postpone this
>> improvement to be after backup-top filter merged.
>
> But do you need to distinguish it? Why not just keep track of memory
> usage and put the current I/O coroutine to sleep in a CoQueue or
> something, and wake that up at the end of backup_do_cow()?
>
1. Because if we _can_ allow doubling of memory, it's more effective to not
restrict allocations on
guest writes. It's just seems to be more effective technique.
2. Anyway, I'd allow some always-available size to allocate - let it be one
cluster, which will correspond
to current behavior and prevent guest io hang in worst case. So I mean, if we
have enough memory allow
individual CBW operation to allocate the whole buffer, and if we have no extra
memory allow to allocate one
cluster.
--
Best regards,
Vladimir
