17.06.2021 18:52, Max Reitz wrote:
As we have attempted before
(https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg06451.html,
"file-posix: Cache lseek result for data regions";
https://lists.nongnu.org/archive/html/qemu-block/2021-02/msg00934.html,
"file-posix: Cache next hole"), this patch seeks to reduce the number of
SEEK_DATA/HOLE operations the file-posix driver has to perform. The
main difference is that this time it is implemented as part of the
general block layer code.
The problem we face is that on some filesystems or in some
circumstances, SEEK_DATA/HOLE is unreasonably slow. Given the
implementation is outside of qemu, there is little we can do about its
performance.
We have already introduced the want_zero parameter to
bdrv_co_block_status() to reduce the number of SEEK_DATA/HOLE calls
unless we really want zero information; but sometimes we do want that
information, because for files that consist largely of zero areas,
special-casing those areas can give large performance boosts. So the
real problem is with files that consist largely of data, so that
inquiring the block status does not gain us much performance, but where
such an inquiry itself takes a lot of time.
To address this, we want to cache data regions. Most of the time, when
bad performance is reported, it is in places where the image is iterated
over from start to end (qemu-img convert or the mirror job), so a simple
yet effective solution is to cache only the current data region.
(Note that only caching data regions but not zero regions means that
returning false information from the cache is not catastrophic: Treating
zeroes as data is fine. While we try to invalidate the cache on zero
writes and discards, such incongruences may still occur when there are
other processes writing to the image.)
We only use the cache for nodes without children (i.e. protocol nodes),
because that is where the problem is: Drivers that rely on block-status
implementations outside of qemu (e.g. SEEK_DATA/HOLE).
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/307
Signed-off-by: Max Reitz <mre...@redhat.com>
---
include/block/block_int.h | 19 ++++++++++
block.c | 2 +
block/io.c | 80 +++++++++++++++++++++++++++++++++++++--
3 files changed, 98 insertions(+), 3 deletions(-)
diff --git a/include/block/block_int.h b/include/block/block_int.h
index a8f9598102..c09512556a 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -832,6 +832,23 @@ struct BdrvChild {
QLIST_ENTRY(BdrvChild) next_parent;
};
+/*
+ * Allows bdrv_co_block_status() to cache one data region for a
+ * protocol node.
+ *
+ * @lock: Lock for accessing this object's fields
+ * @valid: Whether the cache is valid
+ * @data_start: Offset where we know (or strongly assume) is data
+ * @data_end: Offset where the data region ends (which is not necessarily
+ * the start of a zeroed region)
+ */
+typedef struct BdrvBlockStatusCache {
+ CoMutex lock;
+ bool valid;
+ int64_t data_start;
+ int64_t data_end;
+} BdrvBlockStatusCache;
+
struct BlockDriverState {
/* Protected by big QEMU lock or read-only after opening. No special
* locking needed during I/O...
@@ -997,6 +1014,8 @@ struct BlockDriverState {
/* BdrvChild links to this node may never be frozen */
bool never_freeze;
+
+ BdrvBlockStatusCache block_status_cache;
};
struct BlockBackendRootState {
diff --git a/block.c b/block.c
index 3f456892d0..bad64d5c4f 100644
--- a/block.c
+++ b/block.c
@@ -398,6 +398,8 @@ BlockDriverState *bdrv_new(void)
qemu_co_queue_init(&bs->flush_queue);
+ qemu_co_mutex_init(&bs->block_status_cache.lock);
+
for (i = 0; i < bdrv_drain_all_count; i++) {
bdrv_drained_begin(bs);
}
diff --git a/block/io.c b/block/io.c
index 323854d063..320638cc48 100644
--- a/block/io.c
+++ b/block/io.c
@@ -35,6 +35,7 @@
#include "qapi/error.h"
#include "qemu/error-report.h"
#include "qemu/main-loop.h"
+#include "qemu/range.h"
#include "sysemu/replay.h"
/* Maximum bounce buffer for copy-on-read and write zeroes, in bytes */
@@ -1862,6 +1863,7 @@ static int coroutine_fn
bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
bool need_flush = false;
int head = 0;
int tail = 0;
+ BdrvBlockStatusCache *bsc = &bs->block_status_cache;
int max_write_zeroes = MIN_NON_ZERO(bs->bl.max_pwrite_zeroes, INT_MAX);
int alignment = MAX(bs->bl.pwrite_zeroes_alignment,
@@ -1878,6 +1880,16 @@ static int coroutine_fn
bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
return -ENOTSUP;
}
+ /* Invalidate the cached block-status data range if this write overlaps */
+ qemu_co_mutex_lock(&bsc->lock);
+ if (bsc->valid &&
+ ranges_overlap(offset, bytes, bsc->data_start,
+ bsc->data_end - bsc->data_start))
+ {
+ bsc->valid = false;
+ }
+ qemu_co_mutex_unlock(&bsc->lock);
I think this fragment used twice worth small function, like
block_status_cache_discard_region().
That would be clean and avoids duplication..
+
assert(alignment % bs->bl.request_alignment == 0);
head = offset % alignment;
tail = (offset + bytes) % alignment;
@@ -2394,6 +2406,7 @@ static int coroutine_fn
bdrv_co_block_status(BlockDriverState *bs,
int64_t aligned_offset, aligned_bytes;
uint32_t align;
bool has_filtered_child;
+ BdrvBlockStatusCache *bsc = &bs->block_status_cache;
assert(pnum);
*pnum = 0;
@@ -2442,9 +2455,59 @@ static int coroutine_fn
bdrv_co_block_status(BlockDriverState *bs,
aligned_bytes = ROUND_UP(offset + bytes, align) - aligned_offset;
if (bs->drv->bdrv_co_block_status) {
- ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset,
- aligned_bytes, pnum, &local_map,
- &local_file);
+ bool from_cache = false;
+
+ /*
+ * Use the block-status cache only for protocol nodes: Format
+ * drivers are generally quick to inquire the status, but protocol
+ * drivers often need to get information from outside of qemu, so
+ * we do not have control over the actual implementation. There
+ * have been cases where inquiring the status took an unreasonably
+ * long time, and we can do nothing in qemu to fix it.
+ * This is especially problematic for images with large data areas,
+ * because finding the few holes in them and giving them special
+ * treatment does not gain much performance. Therefore, we try to
+ * cache the last-identified data region.
+ *
+ * Second, limiting ourselves to protocol nodes allows us to assume
+ * the block status for data regions to be DATA | OFFSET_VALID, and
+ * that the host offset is the same as the guest offset.
+ *
+ * Note that it is possible that external writers zero parts of
+ * the cached regions without the cache being invalidated, and so
+ * we may report zeroes as data. This is not catastrophic,
+ * however, because reporting zeroes as data is fine.
+ */
+ if (QLIST_EMPTY(&bs->children)) {
+ qemu_co_mutex_lock(&bsc->lock);
+ if (bsc->valid &&
+ aligned_offset >= bsc->data_start &&
+ aligned_offset < bsc->data_end)
+ {
+ ret = BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID;
+ local_file = bs;
+ local_map = aligned_offset;
+
+ *pnum = bsc->data_end - aligned_offset;
+
+ from_cache = true;
+ }
+ qemu_co_mutex_unlock(&bsc->lock);
+ }
+
+ if (!from_cache) {
+ ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset,
+ aligned_bytes, pnum,
&local_map,
+ &local_file);
+
+ if (ret == (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID)) {
You do cache the data for any driver that returns DATA | OFFSET_VALID. But it's
unused for format drivers (which may return OFFSET_VALID of course).
I think, here should be
if ((ret == (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID)) &&
QLIST_EMPTY(&bs->children)) {
+ qemu_co_mutex_lock(&bsc->lock);
+ bsc->data_start = aligned_offset;
+ bsc->data_end = aligned_offset + *pnum;
+ bsc->valid = true;
+ qemu_co_mutex_unlock(&bsc->lock);
+ }
+ }
Hmm, also, new additions may worth a separate small functions too, so that new
object gets a clean API:
bsc_discard()
bsc_get()
bsc_set()
or something like this. I don't insist.
} else {
/* Default code for filters */
@@ -2974,6 +3037,7 @@ int coroutine_fn bdrv_co_pdiscard(BdrvChild *child, int64_t offset,
int max_pdiscard, ret;
int head, tail, align;
BlockDriverState *bs = child->bs;
+ BdrvBlockStatusCache *bsc = &bs->block_status_cache;
if (!bs || !bs->drv || !bdrv_is_inserted(bs)) {
return -ENOMEDIUM;
@@ -2997,6 +3061,16 @@ int coroutine_fn bdrv_co_pdiscard(BdrvChild *child,
int64_t offset,
return 0;
}
+ /* Invalidate the cached block-status data range if this discard overlaps */
+ qemu_co_mutex_lock(&bsc->lock);
+ if (bsc->valid &&
+ ranges_overlap(offset, bytes, bsc->data_start,
+ bsc->data_end - bsc->data_start))
+ {
+ bsc->valid = false;
+ }
+ qemu_co_mutex_unlock(&bsc->lock);
+
/* Discard is advisory, but some devices track and coalesce
* unaligned requests, so we must pass everything down rather than
* round here. Still, most devices will just silently ignore
Overall, seems good to me.
--
Best regards,
Vladimir
