False-sharing with small objects
I'm currently refactoring legacy code, and I stumbled upon this strange node of a linked list
#define CACHE_LINE 128
struct Node {
public:
intptr_t value;
Node *next;
Node *prev;
bool dummy;
private:
// Avoid false sharing
unsigned char padding[CACHE_LINE - sizeof(intptr_t) - 2 * sizeof(Node *) - sizeof(bool)];
};
Assuming that the cache line of the system I work on is 32 bytes wide, that means this object will take up to 4 cache lines (with 32 bytes cache lines). And 3 of them will be completely empty.
This linked list is a part of a lock free queue. I have not determined yet how many producers and how many consumers are using it. From this other post, the answer states the following :
A clarification: for negative consequences at least some accesses to "falsely shared" variables should be writes. If writes are rare, performance impact of false sharing is rather negligible; the more writes (and so cache line invalidate messages) the worse performance.
My question is the following : what degrades the most my performances? False-sharing on this little object, or the fact that a quarter of my cache will be filled with padding ?
Live representation of the memory layout on Compiler Explorer
caching memory-layout false-sharing
asked Nov 22 '18 at 14:52
bl4ckb0nebl4ckb0ne
486520
add a comment |
I'm currently refactoring legacy code, and I stumbled upon this strange node of a linked list
#define CACHE_LINE 128
struct Node {
public:
intptr_t value;
Node *next;
Node *prev;
bool dummy;
private:
// Avoid false sharing
unsigned char padding[CACHE_LINE - sizeof(intptr_t) - 2 * sizeof(Node *) - sizeof(bool)];
};
Assuming that the cache line of the system I work on is 32 bytes wide, that means this object will take up to 4 cache lines (with 32 bytes cache lines). And 3 of them will be completely empty.
This linked list is a part of a lock free queue. I have not determined yet how many producers and how many consumers are using it. From this other post, the answer states the following :
A clarification: for negative consequences at least some accesses to "falsely shared" variables should be writes. If writes are rare, performance impact of false sharing is rather negligible; the more writes (and so cache line invalidate messages) the worse performance.
My question is the following : what degrades the most my performances? False-sharing on this little object, or the fact that a quarter of my cache will be filled with padding ?
Live representation of the memory layout on Compiler Explorer
caching memory-layout false-sharing
asked Nov 22 '18 at 14:52
bl4ckb0nebl4ckb0ne
486520
add a comment |
I'm currently refactoring legacy code, and I stumbled upon this strange node of a linked list
#define CACHE_LINE 128
struct Node {
public:
intptr_t value;
Node *next;
Node *prev;
bool dummy;
private:
// Avoid false sharing
unsigned char padding[CACHE_LINE - sizeof(intptr_t) - 2 * sizeof(Node *) - sizeof(bool)];
};
Assuming that the cache line of the system I work on is 32 bytes wide, that means this object will take up to 4 cache lines (with 32 bytes cache lines). And 3 of them will be completely empty.
This linked list is a part of a lock free queue. I have not determined yet how many producers and how many consumers are using it. From this other post, the answer states the following :
A clarification: for negative consequences at least some accesses to "falsely shared" variables should be writes. If writes are rare, performance impact of false sharing is rather negligible; the more writes (and so cache line invalidate messages) the worse performance.
My question is the following : what degrades the most my performances? False-sharing on this little object, or the fact that a quarter of my cache will be filled with padding ?
Live representation of the memory layout on Compiler Explorer
caching memory-layout false-sharing
asked Nov 22 '18 at 14:52
bl4ckb0nebl4ckb0ne
486520
I'm currently refactoring legacy code, and I stumbled upon this strange node of a linked list
#define CACHE_LINE 128
struct Node {
public:
intptr_t value;
Node *next;
Node *prev;
bool dummy;
private:
// Avoid false sharing
unsigned char padding[CACHE_LINE - sizeof(intptr_t) - 2 * sizeof(Node *) - sizeof(bool)];
};
Assuming that the cache line of the system I work on is 32 bytes wide, that means this object will take up to 4 cache lines (with 32 bytes cache lines). And 3 of them will be completely empty.
This linked list is a part of a lock free queue. I have not determined yet how many producers and how many consumers are using it. From this other post, the answer states the following :
A clarification: for negative consequences at least some accesses to "falsely shared" variables should be writes. If writes are rare, performance impact of false sharing is rather negligible; the more writes (and so cache line invalidate messages) the worse performance.
My question is the following : what degrades the most my performances? False-sharing on this little object, or the fact that a quarter of my cache will be filled with padding ?
Live representation of the memory layout on Compiler Explorer
caching memory-layout false-sharing
caching memory-layout false-sharing
asked Nov 22 '18 at 14:52
bl4ckb0nebl4ckb0ne
486520
asked Nov 22 '18 at 14:52
bl4ckb0nebl4ckb0ne
486520
asked Nov 22 '18 at 14:52
bl4ckb0nebl4ckb0ne
486520
asked Nov 22 '18 at 14:52
bl4ckb0nebl4ckb0ne
486520
asked Nov 22 '18 at 14:52
bl4ckb0nebl4ckb0ne
486520
486520
add a comment |
add a comment |
0
active
oldest
votes
Your Answer
StackExchange.ifUsing("editor", function () {
StackExchange.using("externalEditor", function () {
StackExchange.using("snippets", function () {
StackExchange.snippets.init();
});
});
}, "code-snippets");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "1"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f53433504%2ffalse-sharing-with-small-objects%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
0
active
oldest
votes
0
active
oldest
votes
active
oldest
votes
active
oldest
votes
Thanks for contributing an answer to Stack Overflow!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f53433504%2ffalse-sharing-with-small-objects%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
