Restore a shortcut in hot loop in _mime_hdr_field_list_search_by_string by masaori335 · Pull Request #13119 · apache/trafficserver

masaori335 · 2026-04-27T07:14:43Z

@c-taylor noticed the 10.2.x branch has regression of throughput performance. It looks like several changes made it but the first big drop is introduced by 3b0f0b760387af0c4f6fdd40c4bc98f14dbf0c4d. The shortcut in the hot loop seems important.

Below is comparison of 1 URL against 1 ET_NET thread performance.

commit	req/sec	note
`0868475`	14683.37	(parent of `3b0f0b7`)
`3b0f0b7`	11024.33
`3b0f0b7` + this patch	13535.39

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Pull request overview

This PR restores a fast-path check in the MIME header field linear search hot loop to reduce unnecessary case-insensitive string comparisons and recover throughput regression seen on the 10.2.x branch.

Changes:

Add a name-length equality check before calling strcasecmp() in _mime_hdr_field_list_search_by_string().

bryancall · 2026-04-27T22:17:11Z

What strcasecmp() is it calling and look at inlining the function?

masaori335 · 2026-04-28T01:41:24Z

This strcasecmp in the loop calls our strcasecmp(const std::string_view &lhs, const std::string_view &rhs) in the libswoc, which is inlined.

The problem is that this strcasecmp almost always call libc's ::strncasecmp, because the size comparison inside the function is only done to determine the size_t n passed as the 3rd argument. ( I thought the size comparison does early return in the beginning, but it doesn't )

trafficserver/lib/swoc/src/string_view_util.cc

Lines 29 to 47 in a26a2d8

    
           int 
        
           strcasecmp(const std::string_view &lhs, const std::string_view &rhs) { 
        
             int zret = 0; 
        
             size_t n = rhs.size(); 
        
             // Seems a bit ugly but size comparisons must be done anyway to get the @c strncasecmp args. 
        
             if (lhs.size() < rhs.size()) { 
        
               zret = 1; 
        
               n    = lhs.size(); 
        
             } else if (lhs.size() > rhs.size()) { 
        
               zret = -1; 
        
             } else if (lhs.data() == rhs.data()) { // the same memory, obviously equal. 
        
               return 0; 
        
             } 
        
             int r = ::strncasecmp(lhs.data(), rhs.data(), n); 
        
             return r ? r : zret; 
        
           }

masaori335 · 2026-04-28T06:52:29Z

I introduced a new helper function, bool iequals(const std::string_view &lhs, const std::string_view &rhs), simple equality check case-insensitivly.

I found 3b0f0b7 did the same change in three places, so this PR restores the length check with ts::iequals.

git show 3b0f0b760387af0c4f6fdd40c4bc98f14dbf0c4d | grep -B 10 strcasecmp
 {
   MIMEFieldBlockImpl *fblock;
   MIMEField          *field, *too_far_field;
@@ -1277,8 +1117,9 @@ _mime_hdr_field_list_search_by_string(MIMEHdrImpl *mh, const char *field_name_st
 
     too_far_field = &(fblock->m_field_slots[fblock->m_freetop]);
     while (field < too_far_field) {
-      if (field->is_live() && (field_name_len == field->m_len_name) &&
-          (strncasecmp(field->m_ptr_name, field_name_str, field_name_len) == 0)) {
+      if (field->is_live() &&
+          strcasecmp(std::string_view{field->m_ptr_name, static_cast<std::string_view::size_type>(field->m_len_name)},
--
--- a/src/proxy/http/HttpSM.cc
+++ b/src/proxy/http/HttpSM.cc
@@ -735,9 +735,9 @@ HttpSM::state_read_client_request_header(int event, void *data)
     if (t_state.hdr_info.client_request.version_get() == HTTP_1_1 &&
         (t_state.hdr_info.client_request.method_get_wksidx() == HTTP_WKSIDX_POST ||
          t_state.hdr_info.client_request.method_get_wksidx() == HTTP_WKSIDX_PUT)) {
-      int         len    = 0;
-      const char *expect = t_state.hdr_info.client_request.value_get(MIME_FIELD_EXPECT, MIME_LEN_EXPECT, &len);
-      if ((len == HTTP_LEN_100_CONTINUE) && (strncasecmp(expect, HTTP_VALUE_100_CONTINUE, HTTP_LEN_100_CONTINUE) == 0)) {
+      auto expect{t_state.hdr_info.client_request.value_get(static_cast<std::string_view>(MIME_FIELD_EXPECT))};
+      if (strcasecmp(expect, std::string_view{HTTP_VALUE_100_CONTINUE,
--
@@ -882,11 +888,11 @@ HttpTransactHeaders::remove_conditional_headers(HTTPHdr *outgoing)
 void
 HttpTransactHeaders::remove_100_continue_headers(HttpTransact::State *s, HTTPHdr *outgoing)
 {
-  int         len    = 0;
-  const char *expect = s->hdr_info.client_request.value_get(MIME_FIELD_EXPECT, MIME_LEN_EXPECT, &len);
+  auto expect{s->hdr_info.client_request.value_get(static_cast<std::string_view>(MIME_FIELD_EXPECT))};
 
-  if ((len == HTTP_LEN_100_CONTINUE) && (strncasecmp(expect, HTTP_VALUE_100_CONTINUE, HTTP_LEN_100_CONTINUE) == 0)) {
-    outgoing->field_delete(MIME_FIELD_EXPECT, MIME_LEN_EXPECT);
+  if (strcasecmp(expect,

There're so many other libswoc strcasecmp usages we can replace, but it's out of scope from this PR.

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Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

masaori335 · 2026-04-30T05:46:31Z

+  For case-sensitive comparison, use @c std::string_view::operator==.
+ */
+inline bool
+iequals(std::string_view lhs, std::string_view rhs) noexcept


Standard library doesn't have this, but boost has one. I followed the naming.
https://www.boost.org/doc/libs/latest/libs/beast/doc/html/beast/ref/boost__beast__iequals.html

Does libswoc already have a compare like this?

CaselessEqual is similar, but it's a functor on TextView (won't accept a plain std::string_view). IMO worth adding ts::iequals for simplicity and cleanness.

trafficserver/lib/swoc/include/swoc/TextView.h

Lines 963 to 970 in 4bba678

/// Support for containers that need case insensitive comparisons between views.

struct CaselessEqual {

/// @return @c true if the view contants are equal when compared without regard to case.

bool

operator()(self_type const &lhs, self_type const &rhs) const noexcept {

return lhs.size() == rhs.size() && 0 == strcasecmp(lhs, rhs);

}

};

bneradt

Looks reasonable. Let's merge this in as is.

There's a lot of strcasecmp in our code. Should we more generally replace these?

masaori335 · 2026-05-14T23:26:52Z

I do agree with we should replace all str(n)casecmp() == 0 with this mechanically.

Restore a shortcut in hot loop in _mime_hdr_field_list_search_by_string

4baaf44

masaori335 added this to the 11.0.0 milestone Apr 27, 2026

masaori335 self-assigned this Apr 27, 2026

masaori335 added the Logging label Apr 27, 2026

masaori335 added this to ATS v10.2.x Apr 27, 2026

masaori335 added the Performance label Apr 27, 2026

masaori335 requested a review from Copilot April 27, 2026 09:21

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bryancall requested a review from bneradt April 27, 2026 22:17

Add ts::iequals(const std::string_view &, const std::string_view &)

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masaori335 requested a review from Copilot April 30, 2026 03:52

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Comment thread include/tsutil/StringCompare.h

Address issue from Copilot

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masaori335 commented Apr 30, 2026

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masaori335 requested a review from bryancall May 12, 2026 01:39

bneradt approved these changes May 14, 2026

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masaori335 merged commit e766d7a into apache:master May 14, 2026
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github-project-automation Bot moved this to For v10.2.0 in ATS v10.2.x May 14, 2026

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Restore a shortcut in hot loop in _mime_hdr_field_list_search_by_string#13119

Restore a shortcut in hot loop in _mime_hdr_field_list_search_by_string#13119
masaori335 merged 3 commits into
apache:masterfrom
masaori335:asf-master-fix-regression-0

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	/// Support for containers that need case insensitive comparisons between views.
	struct CaselessEqual {
	/// @return @c true if the view contants are equal when compared without regard to case.
	bool
	operator()(self_type const &lhs, self_type const &rhs) const noexcept {
	return lhs.size() == rhs.size() && 0 == strcasecmp(lhs, rhs);
	}
	};

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