Re: [Qemu-devel] [RFC PATCH V3] qemu-img: make convert async

[Peter Lieven]

 Von:   Kevin Wolf <address@hidden> 
 An:   Peter Lieven <address@hidden> 
 Kopie:   <address@hidden>, <address@hidden>, <address@hidden>, 
<address@hidden> 
 Gesendet:   14.02.2017 17:20 
 Betreff:   Re: [RFC PATCH V3] qemu-img: make convert async 

Am 14.02.2017 um 14:39 hat Peter Lieven geschrieben: 
> this is something I have been thinking about for almost 2 years now. 
> we heavily have the following two use cases when using qemu-img convert. 
>  
> a) reading from NFS and writing to iSCSI for deploying templates 
> b) reading from iSCSI and writing to NFS for backups 
>  
> In both processes we use libiscsi and libnfs so we have no kernel pagecache. 
> As qemu-img convert is implemented with sync operations that means we 
> read one buffer and then write it. No parallelism and each sync request 
> takes as long as it takes until it is completed. 
>  
> This is version 3 of the approach using coroutine worker "threads". 
>  
> So far I have the following runtimes when reading an uncompressed QCOW2 from 
> NFS and writing it to iSCSI (raw): 
>  
> qemu-img (master) 
>  nfs -> iscsi 22.8 secs 
>  nfs -> ram   11.7 secs 
>  ram -> iscsi 12.3 secs 
>  
> qemu-img-async 
>  nfs -> iscsi 12.3 secs 
>  nfs -> ram   10.5 secs 
>  ram -> iscsi 11.7 secs 
>  
> Comments appreciated. 
>  
> Thank you, 
> Peter 
>  
> Signed-off-by: Peter Lieven <address@hidden> 
> --- 
> v2->v3: - updated stats in the commit msg from a host with a better network 
> card 
>         - only wake up the coroutine that is acutally waiting for a write to 
> complete. 
>           this was not only overhead, but also breaking at least linux AIO. 
>         - fix coding style complaints 
>         - rename some variables and structs 
>  
> v1->v2: - using coroutine as worker "threads". [Max] 
>         - keeping the request queue as otherwise it happens 
>           that we wait on BLK_ZERO chunks while keeping the write order. 
>           it also avoids redundant calls to get_block_status and helps 
>           to skip some conditions for fully allocated imaged (!s->min_sparse) 
>  
>  qemu-img.c | 213 
> +++++++++++++++++++++++++++++++++++++++++-------------------- 
>  1 file changed, 145 insertions(+), 68 deletions(-) 
>  
> diff --git a/qemu-img.c b/qemu-img.c 
> index cff22e3..970863f 100644 
> --- a/qemu-img.c 
> +++ b/qemu-img.c 
> @@ -1448,6 +1448,16 @@ enum ImgConvertBlockStatus { 
>      BLK_BACKING_FILE, 
>  }; 
>   
> +typedef struct ImgConvertRequest { 
> +    int64_t sector_num; 
> +    enum ImgConvertBlockStatus status; 
> +    int nb_sectors; 
> +    QSIMPLEQ_ENTRY(ImgConvertRequest) next; 
> +} ImgConvertRequest; 
 
If I understand your code correctly, you first go through the image and 
create an ImgConvertRequest for every chunk with the same allocation 
status (well, actually not every chunk, but that may be a bug, see 
below). 
 
So let's assume a 1 TB image with every other cluster allocated: 
 
    1T / (64k * 2) = 8M 
 
A list with 8 million entries (requiring 8 million mallocs) feels 
relatively large. Not counting malloc overhead (which might be 
considerable here), I think we're at 192 MB memory usage for this case. 
Not as bad as I was afraid, but not really nice either. And 1 TB isn't 
even close to the maximum image size. 
 
Is it really necessary to create this list at the beginning? Why can't 
we just get the next chunk as the old code used to? 


In my initial version with AIO invoking convert_iterate_sectors caused
recursive entries into my queue_fill functions. If I have fixed worker threads
or only a few coroutines for read/write whatsever this might work without.
I will have a look. In my test cases I don't have such heavily fragmented
images. I was at a few hundred entries at most.
The other issue I feared was that the "cache" in convert_iterate_sectors
does not work correctly. But if the calls to this function are still in 
increasing
sector order, it should work.




 
> +/* XXX: this should be a cmdline parameter */ 
> +#define NUM_COROUTINES 8 
> + 
>  typedef struct ImgConvertState { 
>      BlockBackend **src; 
>      int64_t *src_sectors; 
> @@ -1455,6 +1465,8 @@ typedef struct ImgConvertState { 
>      int64_t src_cur_offset; 
>      int64_t total_sectors; 
>      int64_t allocated_sectors; 
> +    int64_t allocated_done; 
> +    int64_t wr_offs; 
>      enum ImgConvertBlockStatus status; 
>      int64_t sector_next_status; 
>      BlockBackend *target; 
> @@ -1464,11 +1476,16 @@ typedef struct ImgConvertState { 
>      int min_sparse; 
>      size_t cluster_sectors; 
>      size_t buf_sectors; 
> +    Coroutine *co[NUM_COROUTINES]; 
> +    int64_t wait_sector_num[NUM_COROUTINES]; 
> +    QSIMPLEQ_HEAD(, ImgConvertRequest) queue; 
> +    int ret; 
>  } ImgConvertState; 
>   
>  static void convert_select_part(ImgConvertState *s, int64_t sector_num) 
>  { 
> -    assert(sector_num >= s->src_cur_offset); 
> +    s->src_cur_offset = 0; 
> +    s->src_cur = 0; 
>      while (sector_num - s->src_cur_offset >= s->src_sectors[s->src_cur]) { 
>          s->src_cur_offset += s->src_sectors[s->src_cur]; 
>          s->src_cur++; 
 
Is this necessary because we can effectively go backwards now when one 
coroutine works already on the next part, but another one is still 
processing the previous one? Worth a comment? 


Exactly that is the problem. I will add a comment.


 
> @@ -1544,11 +1561,13 @@ static int convert_iteration_sectors(ImgConvertState 
> *s, int64_t sector_num) 
>      return n; 
>  } 
>   
> -static int convert_read(ImgConvertState *s, int64_t sector_num, int 
> nb_sectors, 
> -                        uint8_t *buf) 
> +static int convert_co_read(ImgConvertState *s, ImgConvertRequest *req, 
> +                           uint8_t *buf, QEMUIOVector *qiov) 
 
This is a weird interface: You want to get a created, but empty qiov 
from the caller. There's no real point in that. The traditional approch 
is having a struct iovec on the stack here locally and then using 
qemu_iovec_init_external(). 


The idea was to avoid the qemu_iovec every time because this involves a malloc,
but I admit its not nice. Maybe I find a better approach.


 
>  { 
>      int n; 
>      int ret; 
> +    int64_t sector_num = req->sector_num; 
> +    int nb_sectors = req->nb_sectors; 
>   
>      assert(nb_sectors <= s->buf_sectors); 
>      while (nb_sectors > 0) { 
> @@ -1562,10 +1581,13 @@ static int convert_read(ImgConvertState *s, int64_t 
> sector_num, int nb_sectors, 
>          blk = s->src[s->src_cur]; 
>          bs_sectors = s->src_sectors[s->src_cur]; 
>   
> +        qemu_iovec_reset(qiov); 
>          n = MIN(nb_sectors, bs_sectors - (sector_num - s->src_cur_offset)); 
> -        ret = blk_pread(blk, 
> -                        (sector_num - s->src_cur_offset) << 
> BDRV_SECTOR_BITS, 
> -                        buf, n << BDRV_SECTOR_BITS); 
> +        qemu_iovec_add(qiov, buf, n << BDRV_SECTOR_BITS); 
> + 
> +        ret = blk_co_preadv( 
> +                blk, (sector_num - s->src_cur_offset) << BDRV_SECTOR_BITS, 
> +                n << BDRV_SECTOR_BITS, qiov, 0); 
>          if (ret < 0) { 
>              return ret; 
>          } 
> @@ -1578,15 +1600,18 @@ static int convert_read(ImgConvertState *s, int64_t 
> sector_num, int nb_sectors, 
>      return 0; 
>  } 
>   
> -static int convert_write(ImgConvertState *s, int64_t sector_num, int 
> nb_sectors, 
> -                         const uint8_t *buf) 
> + 
> +static int convert_co_write(ImgConvertState *s, ImgConvertRequest *req, 
> +                            uint8_t *buf, QEMUIOVector *qiov) 
 
Same thing here. 
 
Note that convert_write() used to be called unconditionally. This is not 
the case any more because you don't create ImgConvertRequests 
unconditionally. In particular, BLK_ZERO is skipped without -S 0 instead 
of doing a blk_co_pwrite_zeroes(). This is a bug. The assertion in the 
BLK_BACKING_FILE branch of the switch is also dead code now.


You are right. This needs to be fixed.

 
 
>  { 
>      int ret; 
> +    int64_t sector_num = req->sector_num; 
> +    int nb_sectors = req->nb_sectors; 
>   
>      while (nb_sectors > 0) { 
>          int n = nb_sectors; 
> - 
> -        switch (s->status) { 
> +        qemu_iovec_reset(qiov); 
> +        switch (req->status) { 
>          case BLK_BACKING_FILE: 
>              /* If we have a backing file, leave clusters unallocated that 
> are 
>               * unallocated in the source image, so that the backing file is 
> @@ -1607,9 +1632,10 @@ static int convert_write(ImgConvertState *s, int64_t 
> sector_num, int nb_sectors, 
>                      break; 
>                  } 
>   
> -                ret = blk_pwrite_compressed(s->target, 
> -                                            sector_num << BDRV_SECTOR_BITS, 
> -                                            buf, n << BDRV_SECTOR_BITS); 
> +                qemu_iovec_add(qiov, buf, n << BDRV_SECTOR_BITS); 
> +                ret = blk_co_pwritev(s->target, sector_num << 
> BDRV_SECTOR_BITS, 
> +                                     n << BDRV_SECTOR_BITS, qiov, 
> +                                     BDRV_REQ_WRITE_COMPRESSED); 
>                  if (ret < 0) { 
>                      return ret; 
>                  } 
> @@ -1622,8 +1648,9 @@ static int convert_write(ImgConvertState *s, int64_t 
> sector_num, int nb_sectors, 
>              if (!s->min_sparse || 
>                  is_allocated_sectors_min(buf, n, &n, s->min_sparse)) 
>              { 
> -                ret = blk_pwrite(s->target, sector_num << BDRV_SECTOR_BITS, 
> -                                 buf, n << BDRV_SECTOR_BITS, 0); 
> +                qemu_iovec_add(qiov, buf, n << BDRV_SECTOR_BITS); 
> +                ret = blk_co_pwritev(s->target, sector_num << 
> BDRV_SECTOR_BITS, 
> +                                     n << BDRV_SECTOR_BITS, qiov, 0); 
>                  if (ret < 0) { 
>                      return ret; 
>                  } 
> @@ -1635,8 +1662,9 @@ static int convert_write(ImgConvertState *s, int64_t 
> sector_num, int nb_sectors, 
>              if (s->has_zero_init) { 
>                  break; 
>              } 
> -            ret = blk_pwrite_zeroes(s->target, sector_num << 
> BDRV_SECTOR_BITS, 
> -                                    n << BDRV_SECTOR_BITS, 0); 
> +            ret = blk_co_pwrite_zeroes(s->target, 
> +                                       sector_num << BDRV_SECTOR_BITS, 
> +                                       n << BDRV_SECTOR_BITS, 0); 
>              if (ret < 0) { 
>                  return ret; 
>              } 
> @@ -1651,12 +1679,92 @@ static int convert_write(ImgConvertState *s, int64_t 
> sector_num, int nb_sectors, 
>      return 0; 
>  } 
>   
> -static int convert_do_copy(ImgConvertState *s) 
> +static void convert_co_do_copy(void *opaque) 
>  { 
> +    ImgConvertState *s = opaque; 
>      uint8_t *buf = NULL; 
> -    int64_t sector_num, allocated_done; 
> +    int ret, i; 
> +    ImgConvertRequest *req, *next_req; 
> +    QEMUIOVector qiov; 
> +    int index = -1; 
> + 
> +    for (i = 0; i < NUM_COROUTINES; i++) { 
> +        if (s->co[i] == qemu_coroutine_self()) { 
> +            index = i; 
> +            break; 
> +        } 
> +    } 
> +    assert(index >= 0); 
> + 
> +    qemu_iovec_init(&qiov, 1); 
> +    buf = blk_blockalign(s->target, s->buf_sectors * BDRV_SECTOR_SIZE); 
> + 
> +    while (s->ret == -EINPROGRESS && (req = QSIMPLEQ_FIRST(&s->queue))) { 
> +        QSIMPLEQ_REMOVE_HEAD(&s->queue, next); 
> +        next_req = QSIMPLEQ_FIRST(&s->queue); 
> + 
> +        s->allocated_done += req->nb_sectors; 
> +        qemu_progress_print(100.0 * s->allocated_done / 
> s->allocated_sectors, 
> +                            0); 
> + 
> +        if (req->status == BLK_DATA) { 
> +            ret = convert_co_read(s, req, buf, &qiov); 
> +            if (ret < 0) { 
> +                error_report("error while reading sector %" PRId64 
> +                             ": %s", req->sector_num, strerror(-ret)); 
> +                s->ret = ret; 
> +                goto out; 
> +            } 
> +        } 
> + 
> +        /* keep writes in order */ 
> +        while (s->wr_offs != req->sector_num) { 
> +            if (s->ret != -EINPROGRESS) { 
> +                goto out; 
> +            } 
> +            s->wait_sector_num[index] = req->sector_num; 
> +            qemu_coroutine_yield(); 
> +        } 
> +        s->wait_sector_num[index] = -1; 
 
Okay, so may this is the most important part of the patch. 
 
If we need to keep writes in order, we can only have a single coroutine 
that is currently writing. Then how likely is it that more than one 
other coroutine could actually be reading at the same time rather than 
just waiting for the current writer to be finished? 
 
My assumption is that it effectively degenerates to exactly one reader 
and one writer, which means that we don't actually need a command line 
option to configure the number of coroutines, and that our default of 8 
is already higher than useful. 


I will check the result for 4 and 2 coroutines. My observation or fear was
that a read and write that are effectively no-ops (BLK_ZERO with 
s->has_zero_init)
could block the queue and avoid to read ahead.




 
The big question is then if the code could become simpler if we can 
assume exactly two coroutines. Possibly even not two coroutines that do 
the same thing, but one that reads and another one that writes, 
coordinated with the usual coroutine locking mechanisms. (I haven't 
thought enough about it to be sure that it would be an improvement, but 
it doesn't feel too unlikely.) 
 
On the other hand, if keeping writes in order is only actually needed in 
specific cases, other cases could benefit from actual parallelism, even 
though this is not implemented in this patch. 


I tried to disable it. It has had no influence for my test scenario. But with
the current approach such a switch as well as a switch to define the number
of workers could easily be implemented. Having multiple readers (and maybe 
writers)
should increase performance if there is latency involved.


Before I rewrite everything I would propose to try to change the code
to avoid to build the request queue upfront and analyze what a change in number
of coroutines has in the current and the version without request queue?


If we would have a read and a write coroutine and maybe a third (to calculate 
the request
size with convert_iterate_sectors) my idea would be to have a limited request 
queue.
That is passed from coroutine to coroutine.


sth like:
 queue 1: free request object
 fifo queue 2: request objects ready for reading
 fifo queue 3: request objects ready for writing.


The first coroutine would fetch the a free request object and calculate 
nb_sectors and status and
put it into queue 2.
Then the reader coroutine would fetch the next read object in order and perform 
the read. Once
finisched it puts the object into the queue 3.
Then the writer coroutine handles the write and when the write is finished it 
puts it back into
into the freelist (queue 1).


Thanks for your comments,
Peter