On Tue, Jan 07, 2025 at 04:50:24PM -0300, Fabiano Rosas wrote:
> Currently, if an array of pointers contains a NULL pointer, that
> pointer will be encoded as '0' in the stream. Since the JSON writer
> doesn't define a "pointer" type, that '0' will now be an uint64, which
> is different from the original type being pointed to, e.g. struct.
>
> That mixed-type array shouldn't be compressed, otherwise data is lost
> as the code currently makes the whole array have the type of the first
> element.
>
> While we could disable the array compression when a NULL pointer is
> found, the JSON part of the stream still makes part of downtime, so we
> should avoid writing unecessary bytes to it.
>
> Keep the array compression in place, but break the array into several
> type-contiguous pieces if NULL and non-NULL pointers are mixed.
Could I request for a sample JSON dump for an example array in the commit
log? This whole solution looks working but is tricky. A sample could help
people understand (e.g. showing the same "name" being dumped multiple
times..).
Side note: I tried to dump a very basic VM's JSON out to disk, it scares me
on the size:
$ ls -lhS JSON.out
-rw-r--r--. 1 peterx peterx 106K Jan 7 17:18 JSON.out
That's a simplest VM with all default stuff, mostly nothing complex.. I may
really need to measure how the JSON debug strings affect migration function
or perf at some point..
>
> Signed-off-by: Fabiano Rosas <faro...@suse.de>
> ---
> migration/vmstate.c | 33 ++++++++++++++++++++++++++++++++-
> scripts/analyze-migration.py | 9 ++++++++-
> 2 files changed, 40 insertions(+), 2 deletions(-)
>
> diff --git a/migration/vmstate.c b/migration/vmstate.c
> index 52704c822c..a79ccf3875 100644
> --- a/migration/vmstate.c
> +++ b/migration/vmstate.c
> @@ -425,15 +425,19 @@ int vmstate_save_state_v(QEMUFile *f, const
> VMStateDescription *vmsd,
> int size = vmstate_size(opaque, field);
> uint64_t old_offset, written_bytes;
> JSONWriter *vmdesc_loop = vmdesc;
> + bool is_prev_null = false;
>
> trace_vmstate_save_state_loop(vmsd->name, field->name, n_elems);
> if (field->flags & VMS_POINTER) {
> first_elem = *(void **)first_elem;
> assert(first_elem || !n_elems || !size);
> }
> +
> for (i = 0; i < n_elems; i++) {
> void *curr_elem = first_elem + size * i;
> const VMStateField *inner_field;
> + bool is_null;
> + int max_elems = n_elems - i;
>
> old_offset = qemu_file_transferred(f);
> if (field->flags & VMS_ARRAY_OF_POINTER) {
> @@ -448,12 +452,39 @@ int vmstate_save_state_v(QEMUFile *f, const
> VMStateDescription *vmsd,
> * not follow.
> */
> inner_field = vmsd_create_fake_nullptr_field(field);
> + is_null = true;
> } else {
> inner_field = field;
> + is_null = false;
> + }
> +
> + /*
> + * Due to the fake nullptr handling above, if there's mixed
> + * null/non-null data, it doesn't make sense to emit a
> + * compressed array representation spanning the entire array
> + * because the field types will be different (e.g. struct
> + * vs. uint64_t). Search ahead for the next null/non-null
> + * element and start a new compressed array if found.
> + */
> + if (field->flags & VMS_ARRAY_OF_POINTER &&
> + is_null != is_prev_null) {
> +
> + is_prev_null = is_null;
> + vmdesc_loop = vmdesc;
> +
> + for (int j = i + 1; j < n_elems; j++) {
> + void *elem = *(void **)(first_elem + size * j);
> + bool elem_is_null = !elem && size;
> +
> + if (is_null != elem_is_null) {
> + max_elems = j - i;
> + break;
> + }
> + }
> }
>
> vmsd_desc_field_start(vmsd, vmdesc_loop, inner_field,
> - i, n_elems);
> + i, max_elems);
>
> if (inner_field->flags & VMS_STRUCT) {
> ret = vmstate_save_state(f, inner_field->vmsd,
> diff --git a/scripts/analyze-migration.py b/scripts/analyze-migration.py
> index 4836920ddc..9138e91a11 100755
> --- a/scripts/analyze-migration.py
> +++ b/scripts/analyze-migration.py
> @@ -497,7 +497,14 @@ def read(self):
> raise Exception("internal index of data field unmatched
> (%d/%d)" % (len(a), int(field['index'])))
> a.append(field['data'])
> else:
> - self.data[field['name']] = field['data']
> + # There could be multiple entries for the same field
> + # name, e.g. when a compressed array was broken in
> + # more than one piece.
> + if (field['name'] in self.data and
> + type(self.data[field['name']]) == list):
> + self.data[field['name']].append(field['data'])
> + else:
> + self.data[field['name']] = field['data']
Do we realy need these script changes? I thought VMSDFieldStruct always
breaks array_len field into "index" based anyway?
new_fields = []
for field in self.desc['struct']['fields']:
if not 'array_len' in field:
new_fields.append(field)
continue
array_len = field.pop('array_len')
field['index'] = 0
new_fields.append(field)
for i in range(1, array_len):
c = field.copy()
c['index'] = i
new_fields.append(c)
self.desc['struct']['fields'] = new_fields
--
Peter Xu
