[Libguestfs] [PATCH libnbd 8/8] copy: Adaptive queue size
Nir Soffer
nsoffer at redhat.com
Tue Feb 22 11:48:14 UTC 2022
On Mon, Feb 21, 2022 at 5:41 PM Eric Blake <eblake at redhat.com> wrote:
>
> On Sun, Feb 20, 2022 at 02:14:03PM +0200, Nir Soffer wrote:
> > Limit the size of the copy queue also by the number of queued bytes.
> > This allows using many concurrent small requests, required to get good
> > performance, but limiting the number of large requests that are actually
> > faster with lower concurrency.
> >
> > New --queue-size option added to control the maximum queue size. With 2
> > MiB we can have 8 256 KiB requests per connection. The default queue
> > size is 16 MiB, to match the default --requests value (64) with the
> > default --request-size (256 KiB). Testing show that using more than 16
> > 256 KiB requests with one connection do not improve anything.
>
> s/do/does/
>
> >
> > The new option will simplify limiting memory usage when using large
> > requests, like this change in virt-v2v:
> > https://github.com/libguestfs/virt-v2v/commit/c943420219fa0ee971fc228aa4d9127c5ce973f7
> >
> > I tested this change with 3 images:
> >
> > - Fedora 35 + 3g of random data - hopefully simulates a real image
> > - Fully allocated image - the special case when every read command is
> > converted to a write command.
> > - Zero image - the special case when every read command is converted to
> > a zero command.
> >
> > On 2 machines:
> >
> > - laptop: Intel(R) Core(TM) i7-10850H CPU @ 2.70GHz, 12 cpus,
> > 1.5 MiB L2 cache per 2 cpus, 12 MiB L3 cache.
> > - server: Intel(R) Xeon(R) Gold 5218R CPU @ 2.10GHz, 80 cpus,
> > 1 MiB L2 cache per cpu, 27.5 MiB L3 cache.
> >
> > In all cases, both source and destination are served by qemu-nbd, using
> > --cache=none --aio=native. Because qemu-nbd does not support MULTI_CON
>
> MULTI_CONN
>
> > for writing, we are testing a single connection when copying an to
>
> Did you mean 'copying an image to'?
Yes
>
> > qemu-nbd. I tested also copying to null: since in this case we use 4
> > connections (these tests are marked with /ro).
> >
> > Results for copying all images on all machines with nbdcopy v1.11.0 and
> > this change. "before" and "after" are average time of 10 runs.
> >
> > image machine before after queue size improvement
> > ===================================================================
> > fedora laptop 3.044 2.129 2m +43%
> > full laptop 4.900 3.136 2m +56%
> > zero laptop 3.147 2.624 2m +20%
> > -------------------------------------------------------------------
> > fedora server 2.324 2.189 16m +6%
> > full server 3.521 3.380 8m +4%
> > zero server 2.297 2.338 16m -2%
> > -------------------------------------------------------------------
> > fedora/ro laptop 2.040 1.663 1m +23%
> > fedora/ro server 1.585 1.393 2m +14%
> >
> > Signed-off-by: Nir Soffer <nsoffer at redhat.com>
> > ---
> > copy/main.c | 52 ++++++++++++++++++++++++-------------
> > copy/multi-thread-copying.c | 18 +++++++------
> > copy/nbdcopy.h | 1 +
> > copy/nbdcopy.pod | 12 +++++++--
> > 4 files changed, 55 insertions(+), 28 deletions(-)
> >
>
> > static void __attribute__((noreturn))
> > usage (FILE *fp, int exitcode)
> > {
> > fprintf (fp,
> > "\n"
> > "Copy to and from an NBD server:\n"
> > "\n"
> > " nbdcopy [--allocated] [-C N|--connections=N]\n"
> > " [--destination-is-zero|--target-is-zero] [--flush]\n"
> > " [--no-extents] [-p|--progress|--progress=FD]\n"
> > -" [--request-size=N] [-R N|--requests=N] [-S N|--sparse=N]\n"
> > -" [--synchronous] [-T N|--threads=N] [-v|--verbose]\n"
> > +" [--request-size=N] [--queue-size=N] [-R N|--requests=N]\n"
>
> The options are listed in mostly alphabetic order already, so
> --queue-size before --request-size makes more sense to me.
>
> > @@ -104,33 +106,35 @@ main (int argc, char *argv[])
> > {
> > enum {
> > HELP_OPTION = CHAR_MAX + 1,
> > LONG_OPTIONS,
> > SHORT_OPTIONS,
> > ALLOCATED_OPTION,
> > DESTINATION_IS_ZERO_OPTION,
> > FLUSH_OPTION,
> > NO_EXTENTS_OPTION,
> > REQUEST_SIZE_OPTION,
> > + QUEUE_SIZE_OPTION,
>
> Likewise here.
>
> > SYNCHRONOUS_OPTION,
> > };
> > const char *short_options = "C:pR:S:T:vV";
> > const struct option long_options[] = {
> > { "help", no_argument, NULL, HELP_OPTION },
> > { "long-options", no_argument, NULL, LONG_OPTIONS },
> > { "allocated", no_argument, NULL, ALLOCATED_OPTION },
> > { "connections", required_argument, NULL, 'C' },
> > { "destination-is-zero",no_argument, NULL, DESTINATION_IS_ZERO_OPTION },
> > { "flush", no_argument, NULL, FLUSH_OPTION },
> > { "no-extents", no_argument, NULL, NO_EXTENTS_OPTION },
> > { "progress", optional_argument, NULL, 'p' },
> > { "request-size", required_argument, NULL, REQUEST_SIZE_OPTION },
> > + { "queue-size", required_argument, NULL, QUEUE_SIZE_OPTION },
>
> and here.
>
> > { "requests", required_argument, NULL, 'R' },
> > { "short-options", no_argument, NULL, SHORT_OPTIONS },
> > { "sparse", required_argument, NULL, 'S' },
> > { "synchronous", no_argument, NULL, SYNCHRONOUS_OPTION },
> > { "target-is-zero", no_argument, NULL, DESTINATION_IS_ZERO_OPTION },
> > { "threads", required_argument, NULL, 'T' },
> > { "verbose", no_argument, NULL, 'v' },
> > { "version", no_argument, NULL, 'V' },
> > { NULL }
> > };
> > @@ -212,20 +216,28 @@ main (int argc, char *argv[])
> > }
> > if (request_size < MIN_REQUEST_SIZE || request_size > MAX_REQUEST_SIZE ||
> > !is_power_of_2 (request_size)) {
> > fprintf (stderr,
> > "%s: --request-size: must be a power of 2 within %d-%d\n",
> > prog, MIN_REQUEST_SIZE, MAX_REQUEST_SIZE);
> > exit (EXIT_FAILURE);
> > }
> > break;
> >
> > + case QUEUE_SIZE_OPTION:
>
> and here.
>
> > + if (sscanf (optarg, "%u", &queue_size) != 1) {
>
> Matches pre-existing use, but *scanf("%u") has an inherent limitation
> of being non-portable when it comes to dealing with overflow. Better
> is to use the strtol* family of functions when parsing user input into
> an integer - but that fits in a separate patch.
>
> >
> > -/* If the number of requests in flight exceeds the limit, poll
> > - * waiting for at least one request to finish. This enforces
> > - * the user --requests option.
> > +/* If the number of requests in flight or the number of queued bytes
> > + * exceed the limit, poll waiting for at least one request to finish.
>
> Pre-existing difficulty in reading, made worse by more words. I would
> suggest:
>
> If the number of requests or queued bytes in flight exceed limits,
> then poll until enough requests finish.
Nicer, will use this.
>
> > + * This enforces the user --requests and --queue-size options.
> > *
> > * NB: Unfortunately it's not possible to call this from a callback,
> > * since it will deadlock trying to grab the libnbd handle lock. This
> > * means that although the worker thread calls this and enforces the
> > * limit, when we split up requests into subrequests (eg. doing
> > * sparseness detection) we will probably exceed the user request
> > * limit. XXX
> > */
> > static void
> > -wait_for_request_slots (size_t index)
> > +wait_for_request_slots (struct worker *worker)
> > {
> > - while (in_flight (index) >= max_requests)
> > - poll_both_ends (index);
> > + while (in_flight (worker->index) >= max_requests ||
> > + worker->queue_size >= queue_size)
> > + poll_both_ends (worker->index);
> > }
>
> Looks like a nice improvement.
>
> > =head1 SYNOPSIS
> >
> > nbdcopy [--allocated] [-C N|--connections=N]
> > [--destination-is-zero|--target-is-zero] [--flush]
> > [--no-extents] [-p|--progress|--progress=FD]
> > - [--request-size=N] [-R N|--requests=N] [-S N|--sparse=N]
> > - [--synchronous] [-T N|--threads=N] [-v|--verbose]
> > + [--request-size=N] [--queue-size=N] [-R N|--requests=N]
>
> Another spot for my alphabetic sorting request.
>
> > @@ -156,20 +157,27 @@ following shell commands:
> >
> > Set the maximum request size in bytes. The maximum value is 32 MiB,
> > specified by the NBD protocol.
> >
> > =item B<-R> N
> >
> > =item B<--requests=>N
> >
> > Set the maximum number of requests in flight per NBD connection.
> >
> > +=item B<--queue-size=>N
>
> And again.
>
> I'm okay with you making the fixes that we pointed out, without
> necessarily needing to see a v2 post.
I did not notice that the options are mostly sorted. I will sort them
as you suggest before pushing.
Thanks for reviewing!
Nir
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