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Optimize freeMemorySpace() to not loop in vain #200

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1 change: 1 addition & 0 deletions src/MemObject.cc
Original file line number Diff line number Diff line change
Expand Up @@ -100,6 +100,7 @@ MemObject::MemObject()
ping_reply_callback = nullptr;
memset(&start_ping, 0, sizeof(start_ping));
reply_ = new HttpReply;
assert(!repl.data);
}

MemObject::~MemObject()
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48 changes: 47 additions & 1 deletion src/stmem.cc
Original file line number Diff line number Diff line change
Expand Up @@ -13,8 +13,11 @@
#include "HttpReply.h"
#include "mem_node.h"
#include "MemObject.h"
#include "SquidMath.h"
#include "stmem.h"

size_t mem_hdr::ReplPolicyIdleNodesCount = 0;

/*
* NodeGet() is called to get the data buffer to pass to storeIOWrite().
* By setting the write_pending flag here we are assuming that there
Expand Down Expand Up @@ -58,7 +61,9 @@ mem_hdr::endOffset () const
void
mem_hdr::freeContent()
{
const auto initialNodes = size();
nodes.destroy();
updateIdleNodes(initialNodes);
inmem_hi = 0;
debugs(19, 9, this << " hi: " << inmem_hi);
}
Expand All @@ -72,8 +77,10 @@ mem_hdr::unlink(mem_node *aNode)
}

debugs(19, 8, this << " removing " << aNode);
const auto initialNodes = size();
nodes.remove (aNode, NodeCompare);
delete aNode;
updateIdleNodes(initialNodes);
return true;
}

Expand Down Expand Up @@ -320,18 +327,20 @@ mem_hdr::write (StoreIOBuffer const &writeBuffer)
char *currentSource = writeBuffer.data;
size_t len = writeBuffer.length;

const auto initialNodes = size();
while (len && (target = nodeToRecieve(currentOffset))) {
size_t wrote = writeAvailable(target, currentOffset, len, currentSource);
assert (wrote);
len -= wrote;
currentOffset += wrote;
currentSource += wrote;
}
updateIdleNodes(initialNodes);

return true;
}

mem_hdr::mem_hdr() : inmem_hi(0)
mem_hdr::mem_hdr() : inmem_hi(0), removableByReplPolicy(false)
{
debugs(19, 9, this << " hi: " << inmem_hi);
}
Expand Down Expand Up @@ -376,3 +385,40 @@ mem_hdr::getNodes() const
return nodes;
}

static void
UpdateIdleNodesCounter(const size_t oldSize, const size_t newSize)
{
if (newSize == oldSize)
return;
const auto delta = newSize > oldSize ? newSize - oldSize : oldSize - newSize;
const auto oldCount = mem_hdr::ReplPolicyIdleNodesCount;
if (newSize > oldSize) {
mem_hdr::ReplPolicyIdleNodesCount = IncreaseSum(mem_hdr::ReplPolicyIdleNodesCount, delta).value();

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is there a possibility that the total number of pages (to say noting of idle pages) can exceed size_t maximum value?

IMO, yes: Buggy code (elsewhere) may forget to decrease the total (in some cases) or may increase the total by more than the number of pages in mem_hdr.

We could assert that there are no overflows, but that requires approximately the same effort AFAICT.

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Did the IncreaseSum() address your concern?

} else {
assert(mem_hdr::ReplPolicyIdleNodesCount >= delta);
mem_hdr::ReplPolicyIdleNodesCount -= delta;

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I am not sure that 'underflow' is applicable here. How can it happen? We checked that A >= B where both operands are size_t (an unsigned integer type) and then calculate A-B - we must get a size_t C >=0.

Agreed. We are essentially asserting that there is no underflow, and I missed that fact.

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debugs(19, 5, "Updated ReplPolicyIdleNodesCount from " << oldCount << " to " << mem_hdr::ReplPolicyIdleNodesCount);
}

void
mem_hdr::allowedToFreeWithReplPolicy(const bool allowed)
{
if (removableByReplPolicy == allowed)
return;
removableByReplPolicy = allowed;
const auto oldSize = removableByReplPolicy ? 0 : size();
const auto newSize = removableByReplPolicy ? size() : 0;
UpdateIdleNodesCounter(oldSize, newSize);
}

/// Adjusts the ReplPolicyIdleNodesCount counter by the difference
/// between the current size() and oldSize.
void
mem_hdr::updateIdleNodes(const size_t oldSize)
{
if (!removableByReplPolicy)
return;
UpdateIdleNodesCounter(oldSize, size());
}

7 changes: 7 additions & 0 deletions src/stmem.h
Original file line number Diff line number Diff line change
Expand Up @@ -33,6 +33,7 @@ class mem_hdr
void dump() const;
size_t size() const;
mem_node *getBlockContainingLocation (int64_t location) const;
void allowedToFreeWithReplPolicy(const bool allowed);
/* access the contained nodes - easier than punning
* as a container ourselves
*/
Expand All @@ -41,6 +42,10 @@ class mem_hdr

static Splay<mem_node *>::SPLAYCMP NodeCompare;

/// the total number of pages that allowed to be purged by
/// the associated replacement policy
static size_t ReplPolicyIdleNodesCount;

private:
void debugDump() const;
bool unlink(mem_node *aNode);
Expand All @@ -49,8 +54,10 @@ class mem_hdr
bool unionNotEmpty (StoreIOBuffer const &);
mem_node *nodeToRecieve(int64_t offset);
size_t writeAvailable(mem_node *aNode, int64_t location, size_t amount, char const *source);
void updateIdleNodes(const size_t oldSize);
int64_t inmem_hi;
Splay<mem_node *> nodes;
bool removableByReplPolicy; ///< whether nodes allowed to be purged by the associated replacement policy
};

#endif /* SQUID_STMEM_H */
Expand Down
9 changes: 9 additions & 0 deletions src/store.cc
Original file line number Diff line number Diff line change
Expand Up @@ -419,6 +419,8 @@ void
StoreEntry::lock(const char *context)
{
++lock_count;
if (mem_obj)
mem_obj->data_hdr.allowedToFreeWithReplPolicy(false);
debugs(20, 3, context << " locked key " << getMD5Text() << ' ' << *this);
}

Expand Down Expand Up @@ -450,6 +452,9 @@ StoreEntry::unlock(const char *context)
if (lock_count)
return (int) lock_count;

if (mem_obj)
mem_obj->data_hdr.allowedToFreeWithReplPolicy(mem_obj->repl.data);

abandon(context);
return 0;
}
Expand Down Expand Up @@ -756,6 +761,7 @@ StoreEntry::write (StoreIOBuffer writeBuffer)
assert(mem_obj != nullptr);
/* This assert will change when we teach the store to update */
assert(store_status == STORE_PENDING);

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Can we keep this assertion? It feels like it was correct or, at the very least, should not be removed in this PR.

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Restored: this was first moved to isLocalWriter(), but then I forgot to undo after removing isLocalWriter().

assert(locked());

// XXX: caller uses content offset, but we also store headers
writeBuffer.offset += mem_obj->baseReply().hdr_sz;
Expand Down Expand Up @@ -1513,13 +1519,16 @@ StoreEntry::setMemStatus(mem_status_t new_status)
} else {
mem_policy->Add(mem_policy, this, &mem_obj->repl);
debugs(20, 4, "inserted " << *this << " key: " << getMD5Text());
// only idle entries can be freed by the replacement policy
mem_obj->data_hdr.allowedToFreeWithReplPolicy(!locked());
}

++hot_obj_count; // TODO: maintain for the shared hot cache as well
} else {
if (EBIT_TEST(flags, ENTRY_SPECIAL)) {
debugs(20, 4, "not removing special " << *this << " from policy");
} else {
mem_obj->data_hdr.allowedToFreeWithReplPolicy(false);
mem_policy->Remove(mem_policy, this, &mem_obj->repl);

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I think we need a condition (at least for data_hdr.allowedToFreeWithReplPolicy() call): we should skip this call when entry is not in memory policy.

Why?

I assume that if this (used to be IN_MEMORY!) entry is not in mem_policy for some reason, then its removableByReplPolicy flag is already false and calling allowedToFreeWithReplPolicy(false) is harmless. In all other cases, that call will (also) do what it is supposed to do -- decrease the total counter because we are removing the entry from the replacement policy. What am I missing?

In other words, this particular "removal from policy" event alone is sufficient to make removableByReplPolicy false, and our code should express that idea. It is also good to show asymmetry with the true case above it: It takes several events to make removableByReplPolicy true but only one event to make it false.

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Why?

The intention was that if we do something with policy, we need to check that the policy exists (i.e., is configured). However you are right that we can call this allowedToFreeWithReplPolicy() even if the policy has been never configured - it will do nothing in this case.

debugs(20, 4, "removed " << *this);
}
Expand Down
4 changes: 4 additions & 0 deletions src/store/Controller.cc
Original file line number Diff line number Diff line change
Expand Up @@ -534,6 +534,10 @@ Store::Controller::freeMemorySpace(const int bytesRequired)
if (memoryCacheHasSpaceFor(pagesRequired))
return;

// do not free anything if freeing everything freeable would not be enough
if (Less(mem_hdr::ReplPolicyIdleNodesCount, pagesRequired))
return;

// XXX: When store_pages_max is smaller than pagesRequired, we should not
// look for more space (but we do because we want to abandon idle entries?).

Expand Down
1 change: 1 addition & 0 deletions src/tests/testStore.cc
Original file line number Diff line number Diff line change
Expand Up @@ -112,6 +112,7 @@ TestStore::testStats()
Store::Init(aStore);
CPPUNIT_ASSERT_EQUAL(false, aStore->statsCalled);
StoreEntry entry;
entry.lock("TestStore::testStats");
Store::Stats(&entry);
CPPUNIT_ASSERT_EQUAL(true, aStore->statsCalled);
Store::FreeMemory();
Expand Down
1 change: 1 addition & 0 deletions src/tests/testStoreController.cc
Original file line number Diff line number Diff line change
Expand Up @@ -30,6 +30,7 @@ TestStoreController::testStats()
{
Store::Init();
StoreEntry *logEntry = new StoreEntry;
logEntry->lock("TestStoreController::testStats");
logEntry->createMemObject("dummy_storeId", nullptr, HttpRequestMethod());
logEntry->store_status = STORE_PENDING;
TestSwapDirPointer aStore (new TestSwapDir);
Expand Down
1 change: 1 addition & 0 deletions src/tests/testStoreHashIndex.cc
Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,7 @@ void
TestStoreHashIndex::testStats()
{
StoreEntry *logEntry = new StoreEntry;
logEntry->lock("TestStoreHashIndex::testStats");
logEntry->createMemObject("dummy_storeId", nullptr, HttpRequestMethod());
logEntry->store_status = STORE_PENDING;
Store::Init();
Expand Down